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Like the title says, I replaced the motherboard on this computer today, it was identical to the original, sent directly from HP. So I installed it, ate some lunch, came back to test it, and it wouldn't turn on. I double-checked the power supply connector and the power switch connector. It had occurred to me that the problem may lie with the processor, but I can't check it again until tomorrow and was hoping to get some peace of mind tonight by at least having a rough idea of what might be wrong. Can anyone offer some advice?

Have a look here: Tutorial: No POST (Power On System Test)

if you have no responce at all you should also look at the front end connector (power button\reset button) that connects to the motherboard and make sure its not in backward\wrong

Well specifically the Power or Reset button incorrectly orientated will not cause any issues. Leds yes, but Power button no.

does the fans or anything spin up at all

What was your reason for replacing the motherboard? Could well be the power supply instead, they're usually cheapo bestec.

The fans do not spin, and the reason for replacing the motherboard was that the onboard video was bad. The LED on the power supply comes on, but as I'm told, that doesn't always mean it's working.

how did you check the PSU? Did you use a tester tool with a multimeter?
If not, you did not check it properly.

I used a power supply tester today, and everything checked out. I'm thinking that the motherboard is bad, which is horribly inconvenient, having been a replacement already. I'll have a second opinion sometime this week from a technician friend of mine.

Originall you said that you will check the CPU as well (I suppose by replacing?) Did you confirm it's not that?

I nearly forgot to mention it. I did reseat the processor, only to find it was properly placed the entire time. Unless I'm missing something else to check, I'm just about certain it's the motherboard.

Can you just try one more really quick thing

Try unplugging the CPU fan, and then turning it on (for about 5secs say)
I've had fans cause strange issues (Probably a long shot ! )

did you use the same ram or new ram? If the latter, is the ram a compatible brand and type?

So sorry I haven't been around in a while; work has been crazy. To answer Tedster's question, the same RAM from the original motherboard was used. A second opinion by the technician I mentioned confirmed that it was the motherboard and another replacement's on the way. Hopefully that's all it was and the replacement will solve all these problems. Also, since I posted last, I tried a new power supply, but it didn't help.

I've come back to put the issue to rest: the problem was with the motherboard. The replacement that was sent to me was, in fact, defective. The new replacement arrived not too long ago, I installed it, and it worked good as new. I want to thank everyone who contributed and tried to help; I'll be back if I ever need help again.

Thanks for the update