Computer hangs up when in sleep mode

I have a problem I need help with. I have a tower with XP loaded and SP2. I have the power setting set so the monitor goes off in 10 minutes. Most of the time I come back to the computer, hit the space bar and everything works fine. However, on ocassion, I can’t get the machine to wake up and I have to shut down the machine with the power button. Can someone give me a reason why this might be happening? Thanks