Way too vague to even guess.
What starts up?
Does the cd or dvd have flashing light?
Can you hear the hard drive spinning or clicking?
Are you sure the computer is starting or do you just hear the fans blowing?
Do you hear any beeps?
Desktop,Server, or notebook?
Are you sure the monitor works, like trying it on another pc?
Did you open the case and see if the video card is plugged in all the way?
Is the video built into the motherboard, or plugged into a pci slot?
Is your power supply switch on the back on 110 or 220?
Is there at least one memory chip plugged into the main board?
Has the unit worked before and just recently quit, or first time use?
If you want to ship it to Phoenix without the mouse, keyboard, and monitor, I can take a look.
Won't fix it if it's not worth it.