Why SQL Express services is not running in some systems ?

Question

Please help me, 

I done an application in C#.net and the back end is Sql,  when I installed my project in one of my client's system first he can use it smoothly for some months.
But now in his system the sql services are not running, So the application is failed to connect the database.
I tried to start the service manually from windows services but failed.
Is any body know what is the reason for this problem and how can I solve this ?


Expecting your early replay,
Thanking you