My iPad isn't charging
Make sure you are using an official Apple charger.  Sometimes the cheap chargers are hit-or-miss.  I have a generic lightning cable for an iPhone that I bought at a gas station, and sometimes it works, and sometimes it doesn't.

Sometimes it will notice that it's plugged in, but will not actually charge.  As for my knock-off lightning cable, sometimes it depends on which direction I have it inserted (since they fit both ways).

Just play around with it and come back every once in awhile.  If all else fails, take it to a Genius Bar.