I expect you are right, there will not be much wrong.
But the only real approach you have open to you is that standard one when dealing with Broken Things, as you mention.
You need to open it up, visually inspect, make some measurements with a meter to see where juice is getting and where it isn't, and only then can you determine how to proceed.
Anything else is just guesswork.
If it's a variable speed thing, could be the speed controller has died, or the finger operated trigger is not engaging an internal part properly.