|"computer starts up but monitor wont come"|
How do you know the computer starts up? Does it get as far as playing the Windows startup theme music? Does your keyboard appear to be working? Do numlock & capslock keys light up the appropriate LEDs? Will the CD drive open/close?
If the system boots all the way into Windows without any display, and you say you have only onboard graphics then the motherboard has failed.
If as I suspect, the system is not fully booting then you may have any number of things wrong including but not limited to Faulty powersupply, Faulty motherboard, Failed CPU, Bad ram, Faulty optic or harddrive, Faulty PCI card.
Goin' Fishin' (Some day)