If it just dropped its not a problem of TelloFpv. If it flew away or descended without user input thats another story, then I would need to look into this.
But dropping is in no way associated with the app (TelloFpv or any other app). Thats just not how the apps &
tello communicate & work together, there is nothing I can do to cause
Tello fall from the sky. For example the app does not control the motors when landing. Instead it just sends the command "land" to
Tello, and
Tello decends by itself in its own pace, lands, and turns off the motors.
My first idea would be a hardware failure. Dead battery or a bad connection. If it flew OK afterwards on the same battery I'd say a bad connection. Either on the battery / connector or to one motor.