"Error establishing a database connection" on websites using mysql database

All wordpress sites on my server were working fine until I received an email from google saying that it's bots couldn't crawl one or two of the site.   (that a owner didn't pick the issue up first is amazing, but anyway...)
When I try to access a site there is a message "Error establishing a database connection".

On the server, when I try to use phpMyAdmin to see what is going on there is a error connecting to localhost.

After much stuffing about, I can now log into PMA, can see the databases and their data, can see the user accounts that each site should be connecting with yet still have the aforementioned error.

How can I fix this?

FYI:
Windows Server 2008 R2 Standard (IIS 7.5)
MySQL 5.1.66
PHP 5.4
LVL 8
Reece DoddsAsked:
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Reece DoddsAuthor Commented:
something happened to the main database...   It seemed to have lost all of the passwords assigned to the users.  And had lost which users connected to which database.

So all of the connection settings that each wordpress site had it their wp-config.php files was useless.

I added the users, permissions and then changed the passwords for each.
Restarted the server and all is good again.

Not sure how or why this happened, but I've changed the security settings for  MySQL, server remote access and FTP access just in case.
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Dave BaldwinFixer of ProblemsCommented:
I get emails like that from Google now and then.  By themselves, they don't mean much and they certainly can't cause the problem.  

Did an IP address change somewhere?  Are you connecting to 'localhost' using PMA?
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Dave BaldwinFixer of ProblemsCommented:
I did have a problem like that on Godaddy last year.  For a day, their internal DNS went down and the databases were being selected by name.  I changed it over to the IP address and the problem went away.  A few million web sites were pissed at Godaddy for a while for that.
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Reece DoddsAuthor Commented:
self resolved.

Thanks to the members who contributed though.
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