PX 4 crash

My friend’s quad drone crashed with his expensive camera while hovering at 60 Mtrs. He was using PX4. It was very stable as he was in pos_hold however suddenly he got potential thrust warning and quad came down. He shifted to land mode, but there was no response. can we setup any fail safe to ensure that the quad land safely in such conditions or what could be the issue which resulted in this condition. The all up weight was less than 10kgs. The motors used are T motors Mn6007