Diagnosis
Grinding dust and grit have infiltrated the switch mechanism. The dust mixes with grease and forms an abrasive paste that jams the sliding parts.
How to Fix It
The switch must be cleaned or replaced. Housing disassembly is required. Once open, you can access the paddle switch. Blow it out thoroughly with contact cleaner, not WD-40. Work the switch repeatedly to free it. If it's physically damaged, replace the entire switch assembly.
If That Didn't Work
The internal return spring for the switch may have broken or dislodged. This requires opening the switch housing to inspect, which often means replacement is easier.
Heads Up
A sticking switch is a severe safety hazard. Do not use the tool until this is fixed. Consider taping the switch in the off position while working on it to prevent accidental startup.