Sql server 2008 R2 standard installed with default instance. The default instance name is "mssqlserver".
While trying to connect to instance name using servername\mssqlserver from server management studio, an error is thrown and user is unable to connect .
Reason, it turns out to be that you cannot connect to the default instance by its name mssqlserver.For some strange reason, it is not allowed!!!!
You can connect using the server name alone. Or if you are trying from the machine where sql is installed,you can connect to the local instance.Or else if your application demands a "servername\instance format,you could run the sql installer once more and create a named instance