You can use article: http://support.apple.com/kb/TS2802 (it references "iPhone", but is the same for any iOS device)
-Toggle the ringer switch to see if the unit vibrates. If it does, it could be that iPhone is getting power, but is not displaying any image.
-Try turning iPhone off and then on again.
-While connected to the iPhone charger, try to reset the iPhone.
-If the low-battery screen appears, charge the iPhone.
Frozen or unresponsive iPad
Resolve these most common issues:
• Display remains black or blank
• Touch screen not responding
• Application unexpectedly closes or freezes
iPad Frozen? How to Force Quit an App, Reset or Restart Your iPad
What to Do When Your iPad Won't Turn On
iOS: Not responding or does not turn on
iPad: Basic troubleshooting