Are your iPad and the Apple TV connected to the same Wi-Fi?
If they are, a few other things to try:
Restart iPad, Apple TV and router. (Turn them off and on again.)
Turn off Bluetooth on your iPad.
Turn off AirPlay on your Apple TV, and then turn it on again.
Reset iPad, Apple TV and router. On the iPad, press and hold Home and Power button until the Apple logo appears. Release both, your iPad will power up on it's own. On the Apple TV, unplug it for 30 seconds, then plug it in again. Router: turn it off, maybe also unplug it for some time, then plug it in again and restart it.
Hope that helps.