Monitor powered down and wouldn't "wake up"

I'm running WinXP home on a 1.7 Pentium 4, ATI Radeon 9700 w/ 256 RAM.
Last night I left my pc on. The pwere management settings are TurnOff monitor after 15, with hard disk shut down and standby disabled. I've done this before and always "awoke" the pc with a simple jiggle fo the mouse.
This morning I discovered the pc was still on, but the usb optical mouse had turned off. I had to power down the pc in order to get it back. Any ideas what might have caused this?

Thanks

 

Turn off monitor after 15 doesn't appear to be in question. Standby is disabled and will not be a consideration to the problem. However, if the harddisk is shut down, the mouse will not be enabled until the PC is powered up. If you are able to activate the system with a jiggle of the mouse, the machine is not shutdown.

My settings are monitor 10 minutes before off. Standby off. Hibernate after two hours. "Jiggling" the mouse activates the system. There are only 3 options: standby, hibernate or turn off.

I would quess you may be in hypernation (standby is off) mode when you activate by jiggling the mouse.