If your CPU didn't work, the computer would never start. If your getting three beeps, that means your POST is trying to tell you a code. If you've followed all the instructions
On another note, the bare essentials your computer needs to turn on is a motherboard, CPU, RAM and a video card. If you're getting just three beeps, your CPU, RAM, and motherboard are functioning. If it isn't, have you opened the case and modified anything, maybe switch jumpers?
Finally, if you have a video card, make sure that the power dongle is plugged in if it requires one. If you've had the computer for a while, maybe you suffered chip creep. Just push everything in snuggly.
If you've switched out both the CPU and motherboard, I have no idea. Unless you used another board with the same exact model and problem, I dunno. Has your computer overheated before? Does it usually run hot?
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