Few days back we were flying our Tarot 680 copter in Auto mode. We have flown several time with the same setup with no problems. But on that day even though the battery was fully almost completely charged but it was showing in red on mission planner. It completed half of its mission in Survey grid pattern & then suddenly something weird happened. The copter started loosing altitude and finally crashed. After retrieving the copter with fair amount of damage,we tried to power it up to understand what went wrong. But the main Led on Pixhawk was glowing Red indicating Autopilot failure. It was not the first time that day. On the first time same thing happened but nothing bad happened to the copter so we did normal checks and continued with the flight. Can anyone help me figure out what might be the probable cause of failure. I am enclosing the flight log.