Check these two things.
First of all, when you say you had to switch the power on and then off, what is it that are you referring to? I mean, how is your pc connected? Is it on a surge protector or straight to the outlet or some other configuration? Are you switching on and off the PC or a surge protector? Either way, try replacing the power cable and, just so as to avoid any factors, make sure your power cable has three prongs, where one of them is for grounding.
Two, and the more probable culprit, the ATX Power cable to your motherboard is not "clean". It may be defective for some reason or another, or the lead from the front may need to be resoldered. Also, the actual switch on your PSU may not be functioning properly, but I would say that is not the most likely scenario. Still, it reamins a possibility.