At the price
of them these days, why bother to fix one?
But the common problem is the power lead failing usually at or just below the point it enters the body of the tool. Usually way to test is to jiggle the lead with the switch pressed and see if it randomly starts operating. Also, if it's variable speed, it could be a failure in that circuit. If you don't need var speed, it can be by-passed.
One other check (aside from the obvious - the brushes and their tails): The commutator (where the brushes run) is made of segments of copper separated by insulation
. In a dirty environment
, or after heavy use carbon or other conducting material can build up in the gaps where the insulation
is, effectively shorting the tool and preventing it from running. The usual fix is to scrape the gunk out with a thin blade, but CO contact cleaner might work.
If it's anything else and I reckon you just need to pronounce it dead