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Applications stopped communicating with SQL 2005

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
Verified that all services were started.
Named pipes were enabled.
TCPIP were allowed.
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This may be due to some DNS (or firewall) issue - did you tried connecting to it via the IP instead?
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All of our apps connect to the SQL database via IP.
OK, so can you try a telnet from the app server where you get this error to see if the SQL server can be reached?

like in cmd prompt run:

telnet 111.222.333.000 1433

without any : and use the actual port if is not running on default TCP port 1433 - obviously replace 111.222.333.000 with the actual SQL IP

you can also run a command like below by that SQL IP to see where is your network broken link:

tracert 111.222.333.000
This was the first thing we did to rule out the issue.  Port 1433 is open.
OK so if the telnet is working fine to IP port 1433 and SQL is running on it so there's no reason why the app cannot connect to that same IP/port number right? Was there any deployment, or windows update in between the time when this was working and current state?
No updates or deployment.  They just stopped working yesterday afternoon.
I mean you could try create a ODBC on that app server and "test connection" but that will just proof the SQL is not the issue in this case and I think you need to check app config files and maybe try an IISRESET
Modified the app to bypass checking SQL.  The app worked so this showed me that the issue is a connection to SQL.  Checked all of the config files and recycled the app pool as well as rebooted both servers.  (I have a second server with the same apps and I am getting the same error message)
Could you try create a ODBC on that app server using exact same IP/port/credentials and "test connection"?
Also, on the SQL Side  - is there anything error/warning related in the SQL Log files by any chance
you may also try to
Restart your MSSQLSERVER service. sometimes it works. Also, check Instance name(.and /)
have a look here:

hope this will help you.
  • Enable Named Pipes SQL Config Manager
  • Restart the server SQL Config Manager
  • Use proper server and instance names
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