In my rockclimbing days, we carried two loops of 7mm line in our pockets from which we could fashion prussic ascenders. If we found ourselves dangling in mid-air 20' below a cliff, we could laborously, but surely, work our way back up the rope to the edge of the cliff. It was easier to do using a rockclimbing harness, as we were facing the rope, but it should be possible to do this in a treestand harness if you can reach the rope behind you, and if it dangles far enough below you that you can work with it. If the rope dangled all the way down to the ground, you could use the same prussic ascenders to first ascend enough to loosen whatever was holding you to the main rope (probably another prussic knot using larger diameter rope), and then descend to the ground.