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Alright, a little explanation. I have two separate PCs, both of which have virtually an identical installation of SQL Server 2012 Express on them. During installation for both, I had chosen the default options really. Also on both machines, I connect locally, i.e. I connect to the relevant Database using that server's own SQL Server Management Studio. No remote machine connections are done.

Since this morning, I've been having connection problems on one of the two machines. SQL Server Management Studio is no longer able to connection to the SQL Server Express instance that same machine hosts. I keep getting this error:

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: SQL Network Interfaces, **error: 26 - Error Locating Server/Instance Specified**) (Microsoft SQL Server, Error: -1)

I didn't really do any changes, so I don't know why this error has started to come out of the blue! Prior to this error showing, the SQL services that were running (and still are) were the following:

SQL Server (SQLEXPRESS)
SQL Server VSS Writer (SQLEXPRESS)

I have seen a lot of places which recommend turning on the SQL Server Browser service, but I don't think that is the problem because the other machine (which has its own SQL Server 2012 Express installation), doesn't have SQL Server Browser enabled, and it seems to be working fine. But even then, I enabled SQL Server Browser service on the problematic server, and still SQL Server Management Studio cannot connect. Now I get this error:

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: SQL Network Interfaces, **error: 28 - Server doesn't support requested protocol**) (Microsoft SQL Server, Error: -1)

Anybody have any idea what might be causing this ?

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Is there anything relevant in the SQL server ERRORLOG? –  StrayCatDBA Feb 18 '13 at 16:25
    
In Sql Server Configuration Manager, you are absolutely sure SQL Server (SQLEXPRESS) is running on both machines? Perhaps stop it and restart it on the problematic machine to rule out any weird issues that may have happened on the last startup. What is the "Log On As" user listed in SQL Server Configuration Manager (usually it is NETWORKSERVICE), and in Management Studio, are you using SQL authentication or Windows Authentication? Could any passwords have recently changed? –  TTT Feb 19 '13 at 14:52

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