Collapsed. Lift off stones that have been dislodged, and rebuild. Set largest stones at the base of the wall. Arrange all stones so that joints are staggered. Support teetering stones with stone chips.
Heaved by frost. Take down broken section of wall. Dig a trench about 18" deep under wall and fill it half way with rubble. Tamp well. This should provide the drainage necessary to prevent heaving.