We would need more info than this. What was happening just before this problem started? Could it be a flat battery as Laura asks? Never run a battery flat. If you do, charge with the Factory Charger only. It may take up to several hours for your phone to wake up.
Did you drop the phone? Get it wet? Flash something to it?
If I say my side hurts... Would you know I had an appendicitis attack? Or a cramp? Or a case of diverticulitis? Kicked by a horse? Nasty fall? Shrapnel from a landmine?
See our problem trying to diagnose your phone with so little to go on?
If you please, join the site so we can learn more about your device to better help you. If you join you get a reply button and you can keep updating this thread.