When this happens and you unplug the power cord, leave it unplugged for several minutes - try 4 or 5 at first, then try 10 if that does not help. Having no power into the PSU at all for only a short time does NOT completely deprive the mobo of power. The PSU's fully charged capacitors can keep the mobo's minor circuits running for a little while before they really do go dead. Until that happens, any glitch that locked up a mobo loop still has power and can keep it locked up. A completely cold reset of the mobo requires you give it enough no-connection time to let the PSU's capacitors drain down to nothing.
Thanks for the reply. When I moved I didn't power up my computer for weeks and it still did the same thing. I plugged everything back up and it took about an hour before the green light on the back of the power supply came back on.
Couple months ago I installed a TV card to watch TV. I powered down, unplugged everything, computer was off for about 10 minutes, plug everything back up and nothing. About an hour later I was able to power back up.
I'm thinking that Dell must use some kind of screwy power supplies or something. I probably need to just buy a good power supply and install it.