You are going to have to be a little more descriptive.
What kind of computer do you have? (List as much as you know)
What happens when you try to turn it on? (Do the case fans turn on? Can you hear the hard drive start to spin? Does your computer make any beeps?... Again more information is better)
Did any events precede this? (Strange error messages, electrical events, new hardware, new software, you moved your computer, you dropped a drink on it, etc...)
If you let us know more, someone will be able to help you.