Randomly yesterday I was flying. I had marked the home location and had decent GPS 7-11. I was flying in a valley in the mountains. Suddenly without notice the Phantom 2 (running 3.02 started to decend. I quickly throttled up (as I was only 20 feet up). The heli partly landed on a rock with one skid. It was a slow decent. After touching the rock I gained control of roll and maneuvered away from the boulder and it landed on the flat ground. Batt at 32% I switched the battery and moved on. I can only figure the GPS went out and it some how went into land mode as GPS home land was unavailable. What I should have done was switch to ATTI but it was quick and it was my first real issue/emergency. Any thoughts on why this happened and anything to check to make sure this doesn't happen again?