Problem with Gateway not powering up?

It is a Gateway 556ge with an ATX 300 12EB3. One day it was on, I left the room, came back and it wouldn't turn on. I have tested the power supply with a multi meter and it seems to all check out correctly. While the power supply is plugged into the pc, the green light blinks on the back, the fan on the psu didn't run, and another light on the motherboard blinks as well. When the power supply is plugged in while I was checking it, the green light was constant and the fan ran. Could it still be the power supply and if not, what do I check next?