Computer wont boot up

I take it that you are seeing the message "no signal" on the monitor. If this is true, then I would disconnect all the devices inside of the computer case except the CPU and memory. See if you get any video. If you do, then reconnect the devices (hard drive, CD, etc) until you find the one giving you a problem.
If you have done all this and still no video, I would suspect one of two things: Either the power supply or the motherboard.