You've got SQL Server connection problems—or you will, at some point. And of course the error message that you get says something unhelpful. As you troubleshoot, keep in mind these seven symptoms and solutions.
7. The SQL Server service isn't running.
To check the status of the SQL Server service, open the Administrative Tools menu and navigate to Services, SQL Server service, and make sure it's started.
6. The TCP/IP port is blocked by a firewall.
You need to make sure the firewall is configured to allow SQL Server traffic. By default, the SQL Server database engine listens for requests on TCP port 1433, which needs to be open on any intervening firewalls.
5. SQL Server isn't listening on port 1433.
Some organizations change SQL Server's default port 1433 for security reasons. To direct the client connections to the correct port number, configure the TCP port by using SQL Server Configuration Manager or by specifying the port on the applications connection string using the port number keywords.
4. You attempt to connect to a named instance and the SQL Server Browser service isn't running or UDP port 1434 is blocked.
To start the SQL Server Browser service, which provides information about SQL Server instances, use SQL Server Configuration Manager. If you're using default ports, be sure that UDP port 1434 is open on the firewall or specify the correct port for the SQL Server instance you're connecting to in the application's connection string.
3. SQL Server and the client application aren't configured to use the same network protocol.
Using SQL Server Configuration Manager, confirm that both the server and the client computers have at least one enabled protocol in common and check for the network libraries in use. If the server is using TCP/IP but the network client is using the Shared Memory Net-Library, they won't connect. The server and the client must use matching network libraries.
2. You can't resolve the server name.
You can test for network name resolution by using the Ping command with the name of the SQL Server system (e.g., Ping MyServer). If the Ping command fails using the server name, you might have a network name-resolution problem, caused by DNS problems.
1. You can't connect using the IP address.
If everything else looks right, try to connect to SQL Server using the server IP address (e.g., Ping 192.168.100.1). Using the Ping command with the IP address tests basic network connectivity with the server. If the Ping command fails, your problem is with networking, not SQL Server.