I had this happen as well and it's very frustrating. The way I finally got out, and it took a LONG TIME for me to figure out, was to use the method others had for glitching through walls. You roll down hill to the lowest possible point where the rocks meat the grass. crouch down with your back to the rock/border, hit F3 to sitdown indian-style. When you're sitting down try to rotate your character around and after enough tries and good timing you will pop through the rock and back into the world. Good luck!