Cannot reboot/restart computer

I am wondering if anyone else has had this problem and or knows an answer. Out of the blue (like everyother computer problem) my computer will no longer restart when commanded through XP or via my front panel restart button. What I mean by reboot is that it powers down then up but instead of seeing something on my monitors it just sits there and I hear it running with nothing on my screen. Also, if I actually shut down my computer I have to wait over 5 mins to restart it or the same above problem (blank screens) occures. Any ideas as to why? The system was reloaded with XP Pro a few months back and may have existed after reload, I don’t recall.