Windows 7 screen freeze

Could anyone help me? After leaving the computer on for a while, the computer seems to be on still (I can hear the sounds coming from the CPU) but the screen goes blank. (The monitor seems to be inactivated.) When I move/click on the mouse and hit the keyboard, there is no response at all. Any suggestions? I've searched and don't seem to find any answers. I'd appreciated if anyone can provide me a solution or point me to a right place to look. If there is a thread on this subject already, please let me know.
Here are my specs. I am not sure what else I need to provide. Thank you...

Dell Studio 450
Processr: Intel (R)2 Quad CPU Q8300
@2.50GHz 2.50 GHz
Installed memory (RAM): 6.00 GB
system type: 64-biit Operating System
Window 7 Home Premium