"How was it fine before but not now?" It seems pretty clear to me that the "pop" you heard was something moving due to the tension on the cable. Now you are going to have to make an adjustment to take out the slack which that movement introduced. Check all of the places where stops, pulleys, etc. are clamped and secure them to avoid another "pop" and getting stuck in high gear again. Make sure that the cable is not moving within the adjuster and that the adjuster and indicator chain threads are not stripped.