Hi
When you let your battery drain completely and you plug it, sometimes it takes a couple of mins to turn on again, BUT If it's definitely not charging (like you plugged it 10 mins ago and nothing has happened), start from the basics, like check the cable and wall charger, try a different one, make sure they are correctly connected...if after doing all that nothing happens, get an appointment with an Apple genius or go to the nearest apple store, so they can take a look and see what's wrong
Good luck!.