Sounds like a power supply issue. Have you tried unplugging it for about 10 minutes, and try plugging it back in?
What you can do is - power off the machine, unplug it from the wall. Now look on the inside of your case - and unplug and replug in all the molex and other power connectors.
If that doesn't do anything - if you have a spare PSU you could try out, that might work as well.