You haven't given a lot of information to work with, but here goes.
If the phone is completely out of power, you may need to leave it plugged in for 10-20 minutes before anything even appears on the screen.
If you have been charging longer than that and have gotten nothing, are you using the wall charger? Some people, myself included, have had the wall charger units go bad suddenly. Try plugging it into the computer USB port instead.
When plugging it into the computer USB port, be sure you are using a port directly on the computer and that the computer is turned on. Ports on keyboards, monitors, or hubs are sometimes unable to charge your phone, and ports on computers usually will not charge if the computer is turned off.