When my son turns on his computer, the Windows XP start up screen appears (as usual) as if it is loading. Stays like this about 30 seconds and then the screen goes blank, like the computer has been turned off. I'd appreciate any help! Thanks.

Try booting into Safe Mode options:

After the power-on and BIOS POST or your Logo screen repeatedly tap the F8 key or hold the Ctrl key before Windows starts.

Choose to start in: Last Known Good Configuration

Tried it...same blank screen when I chose Last Known Good Configuration...it did work in the Safe Mode, but we want it back the way it was!

Can you think of anything that was done that could have triggered this problem?

You can also try choosing Safe Mode then System Restore to a point prior to this problem.

The next step would be to reload your graphics drivers. To do this you need to ID your graphics device. What is listed in the Device Manager under Display adapters:

go to the Device Manager start Run... devmgmt.msc