Screen Goes Blank on Gateway Laptop

Question

I have a problem with my Gateway M-1625 Notebook which uses Windows Vista Home Basic. The problem is that screen sometimes goes blank right when I'm in the middle of doing something. I can't get it to turn back on. The computer won't go into sleep mode. I've tried leaving the computer alone, and nothing happens. The only thing I've been able to do is to turn off the computer by holding down the power button.This has happened both when I'm running on battery and when I'm using A/C power. I think everytime this has happened was while I was connected to the internet. I originally was using AOL dial-up internet connection and recently switched to People PC (this issue has occurred both before and after I switched ISP).