Computer in black screen, no energy outputted (by 8 users)

A few possibilities(the only problem is if you never get any display to try many of these suggestions):

-Suspend. With suspend, everything is put to RAM and the computer goes to low power state, shutting off USB, HD, and most other things, but the PSU is still on so the fans keep spinning. All it takes to wake it up is either a punch on the keyboard (that is still active) or tapping the power button again.
I saw a computer act like that once except it wasn't really in suspend. It had basically half-crashed, half shut down.

r-emove "extra" components that are not needed for the computer to run. Like Sound cards etc, leaving only the essential components in and see if this fixes it.

-Possibly, one of your attached devices is telling the PC to hibernate/shut down, but it's not happening correctly.
If you go into Device Manager and look at the properties of most devices like NIC, soundcard, etc, you'll see a checkbox for "Allow this device to wake computer up/put it to sleep" etc. Check that out.

-Violent spyware/viruses can also cause these symptoms.

-ACPI, incorrectly implemented at the BIOS level, will not wake from suspend correctly. You would flash your BIOS to the latest version, and then in the power settings in BIOS and in XP, turn off power management (while on AC in XP). If the BIOS does not correctly detect keyboard or mouse wakeup, it will not and appear as it is.
A telltale for lockup (bypassing keyboard/mouse) is if the system does not respond to the power switch. The power switch also is tied into ACPI. A push of the power switch also send a wakeup or suspend/off message. By holding the power switch for 5 seconds, the off is forced through the BIOS (and that is a surer sign that it locked up.)

-A faulty HDD might not wakeup also (ignores or does not get the start signal).

-Also, it is not recommended that you "turn off" ACPI. ACPI APIC adds the benefit of better IRQ handling. If you really can force ACPI "off", you may run into more frequent issues with IRQ conflict resolution (does not mean they will happen, just increases the chances.)

-could be a defective RAM stick, if you get a good one, and replace it one by one with yours, you might find one that when replaced takes away the problem. or run memtest86, or Prime95(on testing - memtest tests ram, prime95 will test cpu/fsb, futuremark will test video,everest can see mobo temps/voltages in windows, a digital multimeter will test a power supply for voltages but it is not 100% accurate because these can change under full load)

-Could be caused by Java issue with Azureus, a bit torrent client.

-Do you have an nvidia card? newer nvidia drivers have issues with java.
Try This:
1. Start->Control Panel->System
2. Click the Advanced Tab
3. Click the Environment Variables button
4. Click the New button
5. For Variable name:_JAVA_OPTIONS
6. For Variable value:-Dsun.java2d.d3d=false
That solves a lot of problems with java it seems, by making it simply not use direct3d for everything.

-updating your motherboard's bios.

-Bad PSU, try switching.

-mobo grounding out.
1. Remove all unnecesary components from mobo, then take it out, leaving it still hooked up to the power source and monitor
2. set mobo on floor,
3. try booting up
4. what happens now?

-Bad mobo DIMM slots.

-It could be something basic including a device driver. Check your eventvwr 's system log and see if the stop was listed. It should at least list the stop number.

I hope something here will clue you on the way to solve your problem, good luck!