I would start by restarting the Apple TV with this process:
If you have Apple TV software 5.0 or later installed, you can restart your Apple TV using the onscreen menu. To restart your Apple TV choose Settings > General > Restart.
I would also try disconnecting from the network, re joining it and trying again:
Disconnect and rejoin your Wi-Fi network by selecting your network on the Apple TV and then choosing Forget Network. Rejoin your Wi-Fi network after it disconnects.
From: Apple TV (2nd and 3rd generation): Troubleshooting Wi-Fi networks and connections
And lastly you can restore the Apple TV with iTunes to see if that resolves the issue:
Apple TV (2nd and 3rd generation): Restoring your Apple TV