First, try just unplugging the computer for 30 seconds or so, and then plug it back in. Sometimes a bigger than normal surge comes down the line (side note: A UPS with voltage regulation will add years to the life of any PC) and trips a failsafe inside the power supply. If you unplug it, let the charge inside dissipate, it will reset. And if it doesn't work, it didn't cost you anything and took about a minute.
I have an Emachine T6216 with XP3.It has been working fine,haven't added any new software or hardware.Had it on a while today and shut it down,went back later to reboot and it booted up fine and while it was loading my programs,all of a sudden my monitor went funny,the desktop wasn't pixilated but all wavy sort of. I had to shut it down with the power button.When I tried to start it up again the only thing that came on were the fans.The fan over the cpu seems to be running very fast and loud,there is no beeping but nothing else happens,the monitor says "no signal".I hooked up another monitor to the computer and that didn't work either.I went inside and took the ram out and put it back ,checked all the connections.
Could it be my motherboard is pooched? I hope I have given enough info for an answer.