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I have a computer, a Dell Optiplex 755, that turns itself on whenever it is being supplied power. It turns on, and all the lights come on, the fans spin, and the monitor does recieve input, but it's just a black screen. I get no noises or beeps or a POST or a BIOS, I get absolutely nothing, just a computer that looks like it should work but doesn't.

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Have you tried using a different video cable? (i.e. HDMI, DVI, VGA) Also, have you tried using a different output on you computer, for example, if you are using a graphics card output, use the motherboard, and if you are using the HDMI, use the VGA. – David Nov 15 '11 at 16:02
Yes, I tried both of them, it's not any problem with the video, the problem is somewhere in the computer itself – Mark Kramer Nov 15 '11 at 16:11
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Are you sure the power supply is switched to the right voltage? Can you test with a different power supply, or use a tester on your current one? Can you try a different monitor or cable? Can you test with a different output? Has this ever worked? If so, what has changed (any new hardware installed)? Have you tried another outlet? Have you moved the computer recently? Have you tried re-seating ram and expansion cards? – MaQleod Nov 15 '11 at 16:13
I took out all the expansion cards, I don't know if anything has changed, I'm the new IT guy here and no one here knows anything about the computer, I have tested with a different outlet, I have tried both different monitors and different cables. However, I haven't tried the power supply. I don't know how to test this one and I don't have a different one to replace it with. – Mark Kramer Nov 15 '11 at 16:16
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When a computer doesn't seem to POST and you've ruled out the extraneous possibilities (display, cable, adapter choice, external power), you're typically left with a few options: video, ram, various motherboard failures, processor, or some odd power supply behavior that affects everything. The trick is then ruling out variables by swapping out/removing them. – MaQleod Nov 15 '11 at 16:26
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Every time i've seen that its been a bad power supply. Look at the diagnostic lights

http://support.dell.com/support/edocs/systems/op755/en/UG/HTML/trouble.htm#wp1302445

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Thank you, I wish I knew about this sooner – Mark Kramer Nov 15 '11 at 18:08
BTW, according to the diagnostics light, it's the memory – Mark Kramer Nov 15 '11 at 18:08
Sometimes all you need to do is simply re-seat the memory. – Rob Nov 16 '11 at 14:20
Also, if you have two (or more) memory modules, you can try just keeping ONE of them at a time to see if the behavior changes. Look at the main board next to the memory slots, there are numbers (DIMM-0, DIMM-1, etc) and you'll want to always start with slot 0. e.g. if you have 2 modules and take one out, the one you keep in should be in slot 0 for testing. Most boards don't like to have a single chip in the other slots. I've had instances where only one of the two memory modules went bad. If you need clarification let us know! – Rob Nov 16 '11 at 14:28
Yeah, I tested the RAM and it was the 4th stick of RAM. It was bad, no matter what combination I put them in, the other three sticks worked fine until I put the 4th one in, then it woulnd't work. So I just took that stick out. It was only 512MB anyway – Mark Kramer Nov 17 '11 at 15:12
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