Monitor display problem

A couple of times I got up in the morning the computer was in hibernation mode, and when I moved the mouse it went to the sign on to windows screen and then the monitor went black. I found if I turned the monitor power on and off it would stay on a second and then go black. I finally got my arrow to click on sign-in and then the monitor stayed on and windows started as usual, and stayed on till I went to bed, and next day. I've been searching on line for answers, but nothing exactly like this out there, hence the post. Could it be a driver update. I'm Not real Computer savvy, so if there were steps to take prior to posting this, please let me know. Thanks in advance for any help.

I have a NVIGIA GeForce 7025/NVIDIA nForce 630a Microsoft XP Home Edition
Version 5.1.2600 Service Pack 3 Build 2600
AMD Sempron Processor 3400+ 1.81 GHz, 1.75GB of RAM