|I've noticed sometimes when a PC is just beeping and you force a shutdown by pulling the power cord it'll often start on its own when plugged back in. Might be a 'last state' startup setting in bios setup.|
As already mentioned 3 beeps is usually a RAM error but you should probably check the beep code for your model to make sure. Have you recently added, changed or removed any ram? Are the ram stick(s) firmly in the socket?
Since the PC isn't booting you can't run software to test the ram. But assuming the problem is RAM then you can remove all but one stick and then see if it boots. Then try a different stick. Try all ram sockets too.