System Specs ?
Mobo, Gfx card, HDD, CDROM, Mem
Initial step in any test is to minimize your hardware set up.
Unplug all drives, The fault could lay with any of these, and put a load on the 12V lines, that stop the PC booting.
Has the mobo got a gfx chip set of its own, separate to the gfx card. Many do, which is ideal for testing purposes, since it allows the gfx card to be removed.
remove any other hardware attached to the machine, apart from the keyboard.
Is the PC speaker connected, or as in some cases there isn't one, then find an old PC speaker and connect to the mobo.
Post code beeps can help immensely when trying to determine a fault
After you minimize, you need to make sure all PSU connectors are inserted correctly.
Switch on and watch the CPU fan and describe what it does and listen to the POST beeps