A few suggestions that have worked for me in the past:
Pull the power cord out of the back of the computer, wait about 30 seconds to a minute, plug it back in, and try again.
Reset the BIOS...can be done by removing the CMOS battery for about 10 seconds, then putting it back in
OR
can be done using shorting the CMOS reset jumper on your motherboard.
You can also try simply flipping the little switch on the back of the power supply to the off position, then back to on.
Hope one or more of these is helpful.