SQL Server Management Studio Express 2012 "ONLY" I have installed SQL Server Express(SSMS) 2012 with management tools. After a few months, the Management studio has stopped working. The error is: 

««
Evaluation period has expired. For more on how to upgrade your evaluation software please go to
http://www.microsoft.com/howtobuy
»»

I dont know why this happened with the express edition. Maybe the Management studio is not released as express edition. What should I do now to get SSMS work with SQL Server 2012?
The earlier releases of SSMS do not work with SQL Server 2012 and I must use SSMS 2012.
It's pretty likely you were using RC0, which expired a few days ago. I would imagine that if you start and stop the 2012 engine service in the services control panel, then it will refuse to start again. 

When you have reinstalled Express it will be fine.