Probably Power Supply
take out the hard drive and turn your pc on, if the power supply is working correctly, you will be in dos and be prompted that there is no hard disk and a beep noise should be heard, from pc speaker.
If nothing happends when you turn it on, most likely the problem is your power supply, it sounds like it is the power supply to me. Do you have another HD tha would boot in the PC, you could try booting anothher HD, to confirm the problem is the HD or the P supply.
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