A quick Google and perusal of a couple of video's suggested that assuming the brushes were in good order, they were, the fine copper wire from one or more of the Stator windings may have broken.
I followed the tear down in this video on Youtube:-
The broken wire can be seen at 2.36 minutes in.
Following the steps in the video I had no problem dismantling the Dremel or in rebuilding.
The broken wire was repaired by unwinding half a turn from the affected winding and after removing the broken wire from the termination clip, see the video, I inserted the wire and reinserted the termination clip. Be careful you do not snap the fine wire again.
Reassemble the Dremel without having any bits left over at the end, Make sure you do not lose the circular steel spring from the rotor shaft lock button in the process.
I am delighted to say the Dremel burst into life when switched on. Job done! For how long remains to be seen.