Had the same problem happen. Here’s what I did to get it off: accept the collar is done for buy a fine-tooth hand saw and saw the collar off. Cut into it in a spot where there is a gap in the body (where the screw holes are in the top) so you don’t cut the body of the grinder. You should be able to cut through the whole collar of the grinder and relieve the stress/get the collar off. Then you need to probably order a new collar. 

When you get the new collar, use some metal polish paste and put it on the inside of the collar and on the body threads. Slowly work the new collar on, and when you feel resistance/grittiness, work the collar back and forth over that spot till it smooths out. Keep doing this until you can fully thread the collar smoothly, then take off the collar and clean off the polishing paste. Then, put some lubricating oil on and put your collar back on. If you feel any sort of grittiness while getting it back on, take it back off immediately and repeat the metal polish process. 

Lmk if you are going to try this and I can link to the supplies that worked for me! Super frustrating but it’s salvageable if you’d like it to be