Why is my motor not working?

Big as in heavy duty and continuous use i presume?

From what it looks like, the carbon brushes could be worn out.

Worn out brushes are basically a more common problem with rotary tools, which can be caused by heavy wear, heat damage, chips, and burrs.

Another problem could be that the device is dirty, here's a bit what ifixit says about that:

Dust and debris can build up inside of the Dremel affecting the performance of the internal parts. Over time this can put stress on the motor and cause it to break down. If the device seems to be dirty internally, remove as much debris as possible and see if issues persist.

But if you just got it since +/- half a year, you should make use of it's 2 year warranty.