I don't know what category this fits into.
I have a Dell Dimension E520 and a Dell Monitor
About a month ago when I play games that demand a high graphics setting( like Crysis or Half-life 2) after about a few minutes the monitor turns off, and the light turns from green to yellow, but the Desktop itself doesn't shut down or go to sleep or whatever.
The Monitor stays like that until I have to restart( hold the off button on the Desktop)
I can still hear the sound from the game but it usually gets stuck.
I have no idea what to do
I am not sure wether this is a virus or software issue.
Could be a hardware or software issue. Open the Windows Event Viewer and look for error messages around the time of a crash. That might point you in the right direction.
Any signs of overheating? Use canned air to blow out all the dust bunnies from inside the case, and make sure all the fans are working, especially the one of the video card, if it has one.
Try uninstalling the video driver in Device Manager, rebooting and installing a fresh download of the latest video driver that's compatible with your hardware and version of Windows. Get it from the manufacturer's site (eg, ATI or nVidia) because Dell tends to be way behind in video drivers.
Have you run any thorough malware scans?
Turns out it is on power save mode
But if i press the settings button on the moniter after it blacked out
it says "In Power Save mode, press computer power button or press keys or move mouse'
How do I turn Power Save off?
Can you test this monitor on a different PC or a different monitor on this PC? If the monitor works on a different PC, then it could be a video issue.
Do you use on-board video or an add-in PCI-e video card? If you have an add-in PCI-e video card, you could move the monitor cable to the output connector for on-board video. Then reboot and press F2 to go into BIOS Setup. Look for the Video option and set it to "on-board", or whatever option it says to use the on-board video rather than an the PCI-e card. Save the change and reboot.
NOTE: You may have to boot in Safe Mode (press F8 at reboot) after connecting the monitor to on-board video to prevent the add-in video card driver from loading. The card's driver won't be compatible with the on-board video. If that works, you can either buy a new video card or remove the video card driver and install the correct drivers for on-board graphics.