The first thing to do is to make sure you have power at the wall outlet. Some power supplies have a switch on the back, check that. It could be the power switch from the case front, but unless you are comfortable with the internal wireing I would leave that up to somebody else. The most logical explanation is the power supply itself. Ask yourself, Did this just happen? or did it give some warning, such as rebooting or strange smells. The best way to test a power supply is with a power supply tester. There is the old paper clip trick, but I personally frown on that approach. Power supply testers are not very much and much safer. If you have to replace the power supply make sure you get the correct form factor and at least as powerfull if not more than the one you are replacing. Remember you are dealing with electricity, so educate yourself and take the proper precautions.