DIAGNOSIS Debris like concrete dust or dried mud has jammed the slide switch mechanism internally.
FIX Remove the battery. Flood the switch area with electrical contact cleaner. Let it soak for a minute. Work the switch back and forth vigorously while wet. Repeat until it moves freely. Allow to fully dry before use.
IF THAT DIDN'T WORK The switch detent spring or the switch itself (part 2 607 001 039) is broken. This requires disassembling the gear housing to replace the switch component.
HEADS UP Do not force the switch with pliers. You will break the plastic housing. The switch must be in the center (lock) position to change the battery.