Many things can cause this, some of which are;
a dead battery
a damaged CPS (Cam position sensor)
a malfunctioning Injector system
a bad starter.
The point is there a lot of things that can cause this particular problem. Be sure to check all the above before taking it up with your auto mechanic.
You can also watch the video about Ford 7.3L Powerstroke Starter Replacement Procedure: