Hi!
Do you have any visible damage, cracks, or the like in the shell? Can you see anything on the outside that is misaligned?
There is a moderate risk that this is not repairable, but if you anyway will be looking for a replacement case, I would suggest attempting to open the charging case to see if you can fix the hinge. The case should not be hard to open. With a bit of luck something has just gotten misaligned and you can pop it back into place. iFixit does not have a proper guide for opening the case, but they do have a teardown you can watch at Youtube where they open the case at 2:35, you can watch it by following this link.
While you work on your device, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same issues with their device. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
If the inside of the case is beyond repair you can always go ahead and buy a replacement case, they're not that expensive.
Please respond with how this went!
Good luck,
Cheers