Monitor doesn't turn on when I move mouse

Like most computers my computer monitors turn off after about 30 minutes of inactivity going into popwer saving mode. Normally when you return to the computer you move the mouse and or press a button on the keyboard to re-turn on the computer monitors and continue working. However today when I moved the mouse nothing happend. The screen stayed blank and did not turn back on. I pressed a button on the keyboard and still nothing. Finally I pressed the power button on the case hoping for the computer to shut down but it did not. When I restarted the computer by turning the power off manually everything apeared to be working fine but can anyone explain to me why moving my mouse today did not allow me to turn on my computer monitor. This is the first time something like this has happend, in fact I doubht it's a windows problem becuase I formatted about a week ago.
Specs
Dimension 8300 Winxp Pro
Vid card Radeon 9800 pro 128mb
1 gig ram
2 HD
250watt OME PSU unit