SQL Server Corrupt database issue

I can't even run DBCC Checkdb on it without getting an error that it can't connect to it.

Is the database online or offline or it has any other status, like Suspect?

It's Offline.

Try to bring it online and then run the DBCC CHECKDB.
If you can't bring it online, provide the error or the SQL Server Error Log.

I can't bring it online.

The error is:

SQL Server detected a logical consistency-based I/O error: unable to decrypt page due to missing DEK. It occurred during a read of page (2:0) in database ID 13 at offset 0000000000000000 in file 'D:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\db_log.ldf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

What happened for the corruption occurs?
You can always try to rebuild the transaction log (you will loose all non committed transactions) by detaching the database and then run the following command:

CREATE DATABASE databaseName
ON (FILENAME='D:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\dbname.mdf')
FOR ATTACH_REBUILD_LOG;

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NOTE: Provide the correct name for the data file (.mdf).

I don't know how the file became corrupted.

I'm unable to detach the database. I get the same errors about I/O error and unable to decrypt the page due to missing DEK.

If you can, stop the SQL Server (it will be hard on Production environments) and then copy the database .mdf file to another place (rename the old one). Start SQL Server and this time the database won't be presented. Now you can run the command I posted before.

I stopped the SQL server. I made a copy of the MDF file and moved it a different location. I renamed the old MDF file. I started the SQL server, but the database is still in the list (but it doesn't say Offline, the icon next to it has a person on the image).

What the following returns?

SELECT *
FROM sys.sysdatabases
WHERE dbid = 13

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NOTE: I used dbid=13 because is the ID presented in the error message. If that's not the correct ID please replace it before running the command.

name      dbid      sid      mode      status      status2      crdate      reserved      category      cmptlevel      filename      version

Db_users      13      0x01      0      69632      1627389952      2016-02-25 17:05:46.627      1900-01-01 00:00:00.000      0      100      D:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\db_users.mdf      655

I don't know that status code. Please check what's the description:

SELECT databasepropertyex('Db_users', 'STATUS')

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The errors also show the same corruption problem with the mdf file.

It says "SUSPECT"

(but it doesn't say Offline, the icon next to it has a person on the image).

Ok, the person means the database is in a Single Mode status.
Stop the SQL Server and rename back the .mdf file to the old one. Start the SQL Server and try to run the following:

USE master
GO

ALTER DATABASE Db_users
SET MULTI_USER
GO

ALTER DATABASE Db_users
SET PAGE_VERIFY CHECKSUM

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Unfortunately, it failed:

Msg 945, Level 14, State 2, Line 1
Database 'Db_users' cannot be opened due to inaccessible files or insufficient memory or disk space.  See the SQL Server errorlog for details.
Msg 5069, Level 16, State 1, Line 1
ALTER DATABASE statement failed.
Msg 824, Level 24, State 6, Line 1
SQL Server detected a logical consistency-based I/O error: unable to decrypt page due to missing DEK. It occurred during a read of page (1:0) in database ID 13 at offset 0000000000000000 in file 'D:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\db_users.mdf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.
Msg 824, Level 24, State 2, Line 1
SQL Server detected a logical consistency-based I/O error: unable to decrypt page due to missing DEK. It occurred during a read of page (2:0) in database ID 13 at offset 0000000000000000 in file 'D:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\db_users_log.ldf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

Actuallly, the little person icon is now gone.

But I still can't access restore or do anything with this existing database.

Hi,skbohler

The MDF file format is very complex. It starts with a file header containing the meta information of the whole file, followed by several parts containing the actual data. You can find more detailed information about the MDF file format by searching on Google with the keyword “MDF file format”. If the file header containing the meta information is corrupt, or any follow-up parts are damaged or corrupted, then you will not be able to open the file correctly with SQL Server. In such a case, the first step is to try to use the built-in repair or recovery function in the SQL Server, to see if it can repair or recover the corrupt file. If not, then you need to resort to a third party SQL Server repair tool to scan the corrupted file and retrieve all recoverable data for you. It is also possible to find a data recovery expert or company to do the task, but in most of the cases, they will also use a third-party tool to do the recovery for you.

The main difference between using the software by yourself and by a third-party service company is:
(1)      The company is familiar with the data recovery industry so he knows which software is the best one. While you need to search and compare the software online by yourself, which is time-consuming.
(2)      The company purchases the software for multiple usages, while you purchase the software for one-time usage only. So normally the service fee may be cheaper than the license fee for the software.
(3)      DIY will keep your data confidential while do the recovery via a company may cause data breach.

If you want to perform a DIY recovery, then you can first use the SQL Server build-in command DBCC CheckDB and CheckTable to repair it, see:

http://technet.microsoft.com/en-us/library/ms176064.aspx

and

http://technet.microsoft.com/en-us/library/ms174338(v=sql.110).aspx

to get more information about the commands. This is the recommended solution which you should try first.

If the corruption is severe and the above commands do not work, then you may try DataNumen SQL Recovery at:

https://www.datanumen.com/sql-recovery/

I have used it in the past and it works well.

Hope this will help. Good Luck!

So you went for option A:

A) Do I have to try and restore from the previous backup (and lose a day of data)?

And lost a day of data, right?

YEs.

Recommendation to close this question by accepting the above comments as solutions.

I agree, updated backup is the beast way to fix any type of database corruption. If corruption is not repairable by using manual ways then, SQL database repair software can be secure way to fix it. Stellar Phoenix SQL Database Repair Software is recommended by Microsoft MVPs.