My son was playing a game on our Dell XPS 2720 when all of a sudden the computer just died. Now when we press the power button, the power button does not light up and it sounds like the only thing that turns on is the fan for the power supply. There is a small green light at the back of the computer underneath where the power cord is plugged in that is lit. Otherwise, there is no other indication of life. I have tried unplugging the machine and holding down the power button for 15 seconds, but that does nothing.
Unplug the system and hold the power button for 30 sec. Remove the back cover and the memory modules -- then reinstall them, button the system back up and try powering up. If that brings it back to life, you're set. If it doesn't, it's a bad mainboard - unfortunately, this is essentially a notebook system on a stand, so the board is proprietary and you'll face a $350-550 charge for a replacement board (plus labor).