How many beeps?
Could be that some power connector has become loose. There can also be a problem with a connection between components within the case.
Remove and reseat any power connectors, remove and reseat any cards installed. If its an older Pc change the motherboard battery (button type)
Do this with the PC unplugged and discharge any static from your person by touching a gounded metal object such as a radiator beofre fidling inside...