This morning my computer started beeping every 30 seconds or so on boot. The monitor doesn't show anything, and my mouse and keyboard don't turn on until after 30 seconds when the first beep occurs (they usually turn on after a couple of seconds during POST). When I press the power button, it turns off immediately, so it doesn't seem to even be getting past the boot stage.
I've tried disconnecting all non-essential hardware, reseating the video card and ram, flashing the CMOS, and nothing seems to make any difference. Everything was working just fine last, and I shut it down normally.
Refer to the sticky on troubleshooting power up issues. It could be several things like a bad PSU, a motherboard that's failing, etc. If you require additional help, please provide detailed info about your system.