theres a little tiny button on the top in the corner. you have 2 press it with your nail. just hold it down for like 2 seconds and it should come back on
there's a little button in the bottom right corner
its a button with a circle in it and a small strait mark in the circle
but it might be in a diffrent place on the phone depending on which type yougot
and you usually have to hold down the button for a few seconds until it turns on