An Athlon of that vintage is likely to be one of the Thunderbird/Palomino/Barton type, they run pretty hot and I wouldn't be surprised if the CPU had overheated . With luck that isn't the case - you could take the side off the machine (unplugged of course) and check for a build up of dust and fluff inside the machine and around the fans. Earth yourself on a metal radiator or something before you touch anything. If it's something less serious, you can try unplugging all drives and extra hardware and try starting the machine. If you can get or borrow a known working power supply you could try that too.