If your armature smells like burnt insulation, or the windings are dark and loose. It sounds like you did fry it.
Since you have only had the tool for such a short time. I'm sure it's not a brush issue. It may be the part number 2610004636 Electronic Speed Switch that is defective. You may want to take it back from where it was purchased to see if you can get a refund or a new tool.
Is this something I can repair, or, do I need to buy a new armature?
My sister borrowed my Dremel 4000 a month or so ago. When I tried to use it the other day it wouldn't come on. She told me that while using it the motor just stopped. I took it apart and nothing seems to be burnt or disconnected. Do you have any ideas what I can do to resolve the issue?
Well without looking at it I could only go with what generally fails on that tool and that's the Electronic Speed Switch
Part Number: 2610004636.Hope that helps.Good luck.
Since the 2610004636 Electronic Speed Switch is the unit that controls the speed of the tool. I would suspect that it is the cause of your problem.
As far as I can see the threads on the 261004634 armature are .28 (pitch) x 40 (threads per inch).
Hope this helps,