I would doubt it is buggered based on that...
Start with the basics and go from there.
Check all external cables are pluged in correctly, then check internal cables. With that case open is there any lights on the motherboard showing when you switch it on?
Is anything starting up, do you hear anything?
If no, after doing the above and you don't have access to a spare PSU (don't watse your money on a new one just yet), then start removing unessential components, e.g. modem, hard drive, gpu, ram see if that makes a difference when you switch the power on, any lights any beeps ,yes tell us what they are. No then start to way up your options buy a new computer, is it worh buying a new PSU for this "very old machine"...chances are you may waste your money on a PSU and it is the motherboard that is shot...
Good luck.