Sigh... I think the others have it well covered.
1. You'll know next time to put everything in writing. Friendships dissolve over smaller things than this, so it's better to be clear up front. Just remember for the next time (if there will be one.)
2. You can demand that the plane be hangared at a FBO nearby. Make sure a mutually accessible FBO has hangarage available, and that the hangar contract is in both yours and his name.
3. Send your ex-friend a certified letter demanding that the airplane be moved to the FBO within 15 calendar days, and that a mutual hangar agreement be signed by both of you for the cost of storage in the FBO hangar. If he does not comply, you will submit the matter to authorities for theft. (Keep the communication where he has forbidden use and access of the plane via prohibition of access to the place where it's kept now.)
4. If he moves the plane to the hangar, make sure you take photos of the Hobbs and equipment to know how much time is on the plane since it is back in mutual control. (To document shared cost of use, etc.)
5. If he does not comply, file a police report and contact an attorney (maybe not necessarily in that order.)
Of course, it will be awkward trying to use the plane if you two aren't communicating. He could be using the plane and landing at other airports "overnight" and it would be out of your reach. So with that in mind, and as Badger and other stated, you can also demand that he buy you out of your share. But you'll want an attorney to get involved to sort that out. (and maybe do steps 2 - 4 above first to demonstrate that you've made all reasonable actions to remedy the situation.)
Of course, this is just my opinion. Your results may (very likely) vary. Good luck.