Could not connect to sql server: SQL server Instance failure. (System.Data)

Question

I get this error message when I try to connect to Sql Server

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Cannot connect to **SQL_ServerName**.

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Instance failure. (System.Data)

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Program Location:

   at System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, Boolean withFailover, Boolean isFirstTransparentAttempt, SqlAuthenticationMethod authType, Boolean disableTnir)

   ...

Answer 1

The issue was I had two SQL server instance on the same machine.

Both instances were listening to connection on the same port, as describe in this forum

Solution 1: Change one instance to run on another port.

1. Open your Sql Server Configuration Manager

2. Go to SQL Server Network Configuration and select Protocols for YOUR_INSTANCE.

3. Right-click on TCP/IP => Properties

4. Change the port from 1433 to another one. I changed it to 1434.

5. Restart sql server and it should work.

Solution 2: Shutdown one sql instance.

1. Go to services

2. Find the instance you want to shutdown.

3. Right-click => Stop

Answer 2

If you have got 2 instances, you take this error. I have MSSQLSERVER and SQL2005 instance name. And after windows server 2008r2 restart, the MSSQLSERVER is working but my default instance SQL2005 is not working. I check the system event log. It says about TCP connection is working another program or service. And I stopped MSSQLSERVER and my default instance SQL2005 started normally... This is port sharing problem.