Management Studio licensing issues
Just uninstall client tools completely, using Control Panel -> Programs and Features -> SQL Server 2012 -> Change. This should launch the setup program and allow you to uninstall the management tools. If the Control Panel method doesn't work, I have a more brute force way of doing it, just make sure you only remove those CLSIDs that relate to SSMS.

Then grab the Express edition of 2012 SP2 (x64 | x86), which has the same functionality as the version you had installed, but without the licensing and the expiration. For more details (or links to more modern versions of SSMS), see this answer.

There are more cumbersome workarounds listed here and here (yes, they say 2008, but they still apply). I think uninstall/reinstall is cleaner.