A couple of months ago my left thumb knuckle (the one closest to the nail) started hurting (deep in the joint)?

the pain increased and progressed down the thumb to the first knuckle. (again, a very deep in the center of the joint kind of pain). Now I can no longer bend the thumb on it's own. Try as I might, I cannot bend the digit. It is still flexible but will not bend on it's own. I can bend it with another finger but it seems like the tendon just quit working. The joint cracks when I bend it which is something new. I'm a guitar playing drummer with the musical saw as my fallback instrument. Playing since age 8 and now I'm 56. I'm bald headed, I'm white and a tad put out by this malady. WTF is going on with my thumb? Anybody care to take a stab at this question? Thank you.