I had this happen once after a drilling cycle (I was running PP 1.9.11). I programmed the RPM way too slow (steel instead of aluminum) and lost some z steps. When the drill cycle was done, the machine tried to overshoot the retract and tripped the switch.
Anyhow next time go into settings to disable the limit switches. Move off the switch, then re-enable them. Re-home and you should be good to go.