I just experienced the same problem with my HP Pavilion desktop this morning, I hadn't used it for a while because I was moving,and last time I used it, it worked fine, but when I plugged it in this morning, power came on, hdd spun, dvd had power, all fans spinning, but no monitor signal or boot up or beeps. It seemed as though the PC wasn't getting enough power to boot.
I thought something might have been knocked loose when it was moved, so I unplugged the hdd cables, video card and power cables to the motherboard quite a few times, checked the ram, though I didn't remove the ram, and took out the battery and then back in again, and I checked for dust but everything seems very clean. After a longwhile of fiddling around with all the connections, Eventually it just started fine for no good reason and went into windows, but then when I tried to restart it it just kept losing power and rebooting at the windows load up, turned it off and it wouldn't boot and no display again, all the time there were no beeps or funny noises. So I pulled one of the ram sticks out completely, started it and away it went, perfect, I tried turning it on an off a few times, and it continued to work well. So then I put the ram back in, and it reads both and it is currently still working fine, even after restarting a number of times. ??!
I can't be 100% sure, but it seems that the ram may have not been contacting properly, although it felt secure and was clipped in properly, others with this problem could try doing the same, and I did use an anti-static strap. It might be that the ram stick was not seated to the motherboard correctly. I thought it was odd as I thought it should have sent a beep code indicating the ram was at fault, but I think that my onboard speaker is not connected, also strange is it went fine with one of the two ram sticks removed, and then after putting it back in they both were reading and I tried turning in on and off again a few times and it seems to be working fine now.
However, I still have a a nagging thought that something other than the ram could have caused it, I am thinking if if one ram wasn't seated properly, it still should have booted up on the other one, as it did when I removed it. If it does the same thing again, I would then assume that the ram or the ram slot on the mb is bad, or the PSU is faulty and not providing enough power. And at the worst, the CPU is faulty or even worse than that the is the motherboard is at fault.
My best guess for now is that the ram was causing an error, and conflicting with the other ram, preventing the PC from booting, after removing it and then reseating it in the same place it removed the error or compatibility issue.