Pull the CPU card and carefully clean the edge connector contacts. I've had good luck with glass foaming cleaner and a microfiber cloth. If you want to clean the socket as well fold some card stock, e. g. 3" X 5" card, soak the folded edge in alcohol and gently insert and remove from slot a few times.

While you have the CPU card out try powering up. I think you'll find it's just like the problem you're experiencing. The x500, x600 series will power up with no CPU or ROM, although it won't do anything afterwards. Power up is controlled from the CUDA chip.