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$