DIAGNOSIS The selector ring is jammed by debris or a damaged detent spring/ball inside the gear housing.
FIX Clean externally first. Remove battery. Use a stiff brush and dry air around the selector ring base. Attempt to turn the ring while gently pulling the trigger in short bursts (no battery) to slightly move internal gears. If it frees up, work it back and forth and clean thoroughly.
IF THAT DIDN'T WORK Internal damage. A common failure point is the nylon selector cam or the steel detent ball/spring behind it. This requires disassembly of the front gear housing to inspect and replace the damaged components.
HEADS UP Do not force the selector ring with pliers. You will shear the plastic cam inside, necessitating a full gear case teardown and replacement of multiple parts.