Make sure your keyboard and mouse and video cable are plugged in tightly, also hard drive cable and power to the hard drive.
Sometimes you have to re-set the bios. If you have a manual you can look there. You will find, on your mobo, 3 pins and there will be a jumper on 2 of these pins, and you should find the word "cmos" right beside the pins.
If you have found that then take the battery out, then move the jumper so that it moves over one pin, so you take the jumper off and put it on the 2 end pins on the other side.
... ...
original move jumper to last 2 pins
jumper is on
first two pins
Leave it that way for a half hour, then move the jumper back to its original position, on the first two pins, then put the battery back in and check all your cables and then start up the computer.
Make sure monitor is turned on too.
If that does work then you may have to go into bios and change some settings back to where they were before the problem began.
Also, you need to let us know what make and model motherboard you have, and other info like what kind of cpu, etc.
Do you have a manual for the mobo?