Have you tried an absolute bare startup, using nothing except CPU and memory? Unplug everything from their slots, and I mean everything. No video, sound, drives, or anything. Just run it totally bare, and if your PC speaker gives you a beep, then your problem is with a component. If it doesn't, you've got a break in your BUS. Could be bad CPU, bad memory, or bad motherboard, It matters not. If your memory is bad (which I doubt), your CPU and motherboard can be saved. Otherwise, it's best to replace every system component, including the PSU, as the CPU, Motherboard, and PSU can all contribute to a global hardware failure.
Anyway, before you do that, make sure the jumper/switch is not set at the reset position. That would definately prevent it from booting at all. It's a simple thing to check, and trust me, I have been foolish enough to overlook it more than once

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