[q=Spuuky]Your power supply could have been switched from from one power level to the other. Try changing the little switch on it (usually red, between the place where the power cable plugs in and the actual power switch).[/q]

You may not want to try that unless you are absolutely sure that's what has happened. Using the wrong power units more often than not will blow your power source up (literally) which results in a not very usable computer.