Here are the short-form answers.
- Plug It In, Let It Charge — And Wait
 - Hold Power + Home to Perform a Hard Reset
 - Restore the iOS Operating System With iTunes
 
The full article at HowToGeek.com explains all the hows and whys of each step.

Here are the short-form answers.
The full article at HowToGeek.com explains all the hows and whys of each step.