Is it only starting in safe mode? If not, don't worry about it.
If it is, the first thing to try is to run system restore to restore it to an earlier configuration that, hopefully, will work. You can do so even in safe mode command prompt only mode by following [https://support.microsoft.com/en-us/kb/304449|these instructions from Microsoft.]
If that doesn't do it, you may have [https://support.microsoft.com/en-us/kb/917964|to perform a repair installation as outlined here.]