Typically, the SQL Express instance is named SQLEXPRESS, so unless you changed this during installation, you need to change the instance name you're connecting to. You can use the instructions here to verify your instance name and port number. Also, those instructions will help you verify that TCP\IP protocol is enabled.
You should confirm the instance name and port number you're running on, then you can do the following:
- If you are using port 1433 then you don't need to provide the instance name, server name will be enough as the native client tries 1433 by default when no port or instance name is specified.
- If you are not using port 1433, and the SQL Browser service is running, specify the server name in the format SERVER\INSTANCE
- If you are not using port 1433, and the SQL Browser service is NOT running, specify the server name in the format SERVER,PORT or SERVER\INSTANCE,PORT. Instance is not necessary when specifying the port number, but you can add it if you want.
The instance name is simply a user-friendly identifier that the native client uses to determine which port number to connect. When connecting, if an instance name is provided, it first polls SQL Browser on UDP port 1434 to obtain the port number for the provided instance name, then reconnects to the server on the port identified by SQL Browser.
By specifying the port number, the check against SQL Browser is skipped and the client simply connects directly on that port. If no instance name is supplied, the client tries to connect on the default port of 1433.