My laptop just completely stopped working. Help?

This morning I had a browser open, as well as MSN, when, without warning, no pop ups, no blue screen, my laptop just closed out of itself.
I opened it again, i chose to start Windows (Vista) in normal mode and that worked.
Opened MSN and Opera again and after a few minutes the same happened.
Now the laptop won’t even start…, the on/off button just flickers on and off.
Can somebody tell me what this is, what causes it and what i can do to possibly fix this?
Thanks in advance!