|Diagnosing a non-starter requires a systematic approach. You have begun well by trying a known good powersupply.|
Failure to boot can be caused by a number of things including failed CPU, failed graphics card, faulty ram, faulty hard drive, faulty CD drive, faulty floppy drive, failed motherboard or even a faulty PCI card such as network card or soundcard.
Begin by disconnecting the power and data cables to all drives. remove all but one stick of ram. remove any add-in cards including graphics card (unless the motherboard does not have an on-board graphics port). remove any/all USB devices.
Leave only the power connections to motherboard.CPU with heatsink/fan. 1 stick of ram, monitor connected to onboard graphics if available, and keyboard.
Now connect power and see if you can start up and get into BIOS.
If you can then you know one of the peripheral devices is the problem. If you cannot get it to boot in barebones mode then either the CPU or motherboard has failed.
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