There are 4 things to check if Apple TV isn't working:

Check your iPad:

1- Check to make sure your iPad is on the MPS-Apple network (Settings, Wi-Fi)

2- Check to make sure your iPad's software is up to date. Go to Settings, then, General, then Software Update. If you need to update it, do so (it will take a few minutes).

Check your Apple TV:

3- Check to make sure AirPlay is on for Apple TV (turn on your projector and Apple TV, use the remote control to go to Settings from the Apple TV home screen, then go to AirPlay. It should say on. If not, turn it on).

4- Check if Apple TV has a software update. Again, turn on your projector and Apple TV, use the remote control to go to Settings from the Apple TV home screen, and then go to General, then Software Update. If there is an update, the device will download and install it. This can take a long time (at least 10 minutes).