Try the following steps, check whether things are working after each step where appropriate, before trying the next.
- Check AirPlay is turned on on the Apple TV (turn it off and on if it already is)
- Check that both devices are on the same network (Settings > Wifi, on the mobile device and Settings > General > Network, on the Apple TV).
- Restart the Apple TV (Settings > General > Restart).
- Restart the Apple TV by removing ALL the cables for 30 seconds.
- Restart your router.
- Restart your mobile device.
I had the same problem. I was stupid to upgrade my atv2 and the airplay icon dissapered after 1.5 hours. Rebooted the router and it worked for another 1.5 hour. I was going crazy! After 1 year of perfect airstreaming and now this.
This seemed to work: (going in my third hour now with airplay)
I had my 5GHz wifi-connection on atv2 and macbook air. 2.5 GHz on my iPhone.
Changed atv2 and macbook to 2.5GHz and ts still running.
This is stupid because I want the 5GHz for all my videowatching!
I hope Apple solves this and I will upgrade once more then NEVER again!