Screen remains black

Hey,
yesterday we've been playing some Counter-Strike as per usual. We stopped at 3AM, I've turn off the computer and went to sleep. Just to remind, there were NO problems the whole day. Today as I woke up I started the computer, but the screen remained black. The controller lights blue, which means there is incoming signal, but it's all black. I've tried to turn off and on the monitor but the screen appears on just 2 seconds and the left top corner is darker than usual. After 2 seconds it goes black again.
Any suggestions?
Thanks.
PS: Happy new year.