Full specs please and make and model of the pc. Sounds like it might be a graphics card problem. Do you have a separate graphics card or is graphics built into the motherboard? You can try another graphics card in there if you have one available that you can swap out. Also, make sure your cabling to the monitor is good. For a general troubleshooting approach you can try the out of case build procedure described here:
http://www.pcmech.com/forum/build-yo...build-try.html
It helps limit things down to the bare minimum so you can better isolate what is going on. If the pc is made by emachines, suspect a bad psu - they are notorious for using cheap psus that blow and take out the motherboard with them.