DIAGNOSIS Yeah, this one's common. The variable speed dial gets jammed with fine dust (especially drywall or wood dust) that works its way into the potentiometer behind it, preventing electrical contact.
FIX First, try to mechanically free it. Remove the battery. Turn the dial back and forth through its full range as firmly as you dare, maybe 20-30 times. This can sometimes grind the dust down and re-establish contact. If that works, follow up by blowing compressed air around the base of the dial to clear out the debris.
IF THAT DIDN'T WORK The potentiometer itself is likely failed. It's a sealed electronic component soldered to the main board. You can't clean it, so the whole trigger/speed control assembly needs replacement, which is a job for an authorized service center.
HEADS UP This is a weak point on this model. When you're done using it, don't just toss it in the bag. Give it a quick blow-off with an air compressor or a stiff brush around the dial and trigger to help keep the dust out.