It could be that the capacitors went bad.
Or power supplies quit working properly.
My sister's XP came on but that's it. Nothing on the screen at all. I heard and felt the fan and the light came on.
That might be due to a weak power supply that failed to initialize the video card at boot. I had that problem with an old Dell. Sometimes if you leave it running a while, maybe 30 minutes, then reboot, it will eventually initialize the video properly, but that doesn't always work.
My 98 isn't doing anything at all. No lights, no noise, no nothing.
Make sure that there isn't a toggle switch on the power supply on the back of the computer that you forgot about. (this is something I've done)