I just recently bought a new case and transferred my computer into it. I had a little trouble with the front panel motherboard connections because I have a dell proprietary mobo and there wasn't a manual that i could find. Anyway, after a bit of tinkering i got the power button and LED's to work. My computer booted up normally, though the front panel USB and audio connectors weren't being detected. When I then turned off my computer, the power supply and all the fans never turned off (even though the computer was shut down). I had to manually flip the switch in the back of the psu. Now the computer won't turn back on at all (the front panel LEDs still work). I've tested the PSU on another computer and it seems to work. Does this mean I have a dead motherboard on my hands or is there some other possibility I haven't thought of?
the computer probably didnt "shut down" when you turned it off. (Did you turn it off via the power button or via the os?)
And in doing so probably entered a sleep state or at least tried to. If in that case you will have to removed the power cord from the power supply and allow the motherboard to leak down its volatge and try to boot again.
If it does boot, watch to see if when booting it say's "resuming windows". If it does then it did try to sleep and wrote its resume data to the drive and either hung, or went into S1 power state... which would explain the fans.
If removing power from the power supply then trying to boot again fails... then you might need to reset the bios. There will either be a small micro button or jumper to do so. After which would clear the bios from thinking its asleep and allow a cold boot.