There or no failsafe functions stand today,
If the Bluetooth Controler disconnect if been used or the App crashed or the phone shutdown the Trello will follow the last action.
It happend to me a few days ago. I had my iphone on an external battery but forgot to turn it on so after a few minutes my phone died. The tello stopped moving and kept hovering. I quickly turned on the battery and restarted my phone and restarted the tello app. The controls came back and I was able to fly again. Maybe if it took longer it would have landed automatically? Don't know, I was to scared to wait any longer
I guess the steps is :
1. Run as fast as possible exactly below your Tello
2. Check your phone and connection:
a) If it's dead, charge it real quick then re-open Tello app.
b) If the connection dropped, reconnect the wifi then re-open Tello app.
3. If none above can't be one, wait 1 minute. It will auto land. Make sure it's not above the water.
and how If you're Tello lands above water?
Spend another 99$