How do i fix my computer if i cannot turn it on?

Its not an easy fix, the easiest method of course as mentioned is reformatting it. The hardest method would be using a live CD. http://www.livecdlist.com/ and download that on a seperate computer burn to cd. Put in your computer, just hit the CD eject as your computer turns on. Put it in, close it, then restart again. Boot from CD. You will now be running an operating system from your CD drive expect it to be slow. Then fix what ever problem or copy off the data you want to recover on to a external drive. Then either fix the problem or ideally reformat.