In my case, I Installed SQL Express 2012 and the problem raise after I reboot my PC. I solved so:
I go to Services -> SQL Server (SQLEXPRESS) and I see that was stopped. I started and It's say an login error. In properties, Tab Log On, I change it to Local System account , then I can start the SQL service.
Then to test it, I go to SQL Server Management Studio and I connect to .\SQLEXPRESS and it works!