Anything can be a problem, you have to go through the logs and check for errors. Though my bet is some software you upgraded is casing a problem. Though it can be more things. Get into administrative runlevel and get the logs. Than analyse them and you will solve your problem in no time.
Anyways have you checked for HCL
of your O.S.? Does your hardware meet the requirements for that O.S.?
If not than find an O.S. that is compatible with your hardware and reinstall the whole system, but first sure make sure you backup your data so you can restore them on your new O.S.