I've had a computer sitting dormant for a few months, it's all hooked up and I was using it at one time. Recently I tried firing it up cause I needed something on it and when I booted it up, the monitor turned off after the loading screen. This happened several times after many restarts. I tried booting in safe mode while I could see the screen, and checked and adjusted my display settings. It worked in safe mode, but no matter what I did I couldn't get it working on a normal boot. I gave up on it for a little while and today I tried switching monitors, I tried other outputs, there are 3(1DVI, and 1 Analog on a card, and 1 Analog on the board), anything I could think of. I can't get anything now. It doesn't even come on when I first power up the PC anymore. What can I do to see something on any screen so I can redo whatever needs to be redone? I can't even change the settings on the monitor without it recognizing a computer.

The answer will have to depend on what kind of windows your using;
Secondly if you can get it up in safe mode then try uinstalling the graphic drivers

1) go to control panel
2) look for system /system properties
3 ) look for device manager usually found in hard ware tab (Annoyances.org - Go Directly to Device Manager (http://www.annoyances.org/exec/show/article01-420)) (Device Manager - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Device_Manager))

4) look for display adapters click on the + sign this will show you the name of the grphic card installed right-click it & select uninstall
5)reset your PC & start in normal mode !

This should solve your problem !

(Since this way the computer will std graphic driver provided by windows which works with all kinds of monitors)

On said computer I'm running XP Pro, and I wish I could get into safe mode. I was able to do it once before, but I can't now. When I turn on the computer the display does not turn on at all, or it doesn't come out of standby I mean. I've tried the other outputs, I've tried othe monitors.

Tell me one thing can u see the Cmos start/computer logo/ or memory testing phase (called the POST screen)?

This usually shows up before windows Xp logo comes

Neither, I don't see anything, when I turn on the computer the monitor does not come on. Any monitor.

Have you reset the BIOS settings recently? If you've got onboard video on the motherboard, and the BIOS was reset, it may be set to use the onboard instead of an AGP or PCI card.

Other than that, if you know that they monitor works (try it on a different computer), then the video card may have gone bad. I would open the case up and make sure the card is in there nice and tight, and that there's a screw holding it in the slot. Try booting up again, and if it doesn't work, try resetting the BIOS (there's usually a jumper on the motherboard for this), and see what it does.

Common causes for a computer not booting to the POST screen are a fauly CPU, faulty RAM, a bad video card, or incorrect BIOS settings.