I broke my wrist and had two different splints on for a week apiece  thne DR put me in a cast for two weeks.  cast is off now, and I have no movement in my thumb at the knuckle--I cannot bend it-it rotates but doesnt bend--previous to this cast, which really hurt, there was noting wrong with my thumb
Usually when you don't move a joint for a long time, stiffness happens to this joint. You need to gradually try to move to with the help of hot fomentation. If no significant improvement happened, you may need some physiotherapy.