Maybe not the most miraculous recovery I've seen, but one that sticks vividly in my memory for how unusual and rewarding it was:

During residency, I was rotating on the general med wards and one of my patients was a little old lady who was completely catatonic. Every day that I would go in to talk to her and examine her, she would be completely speechless and motionless in bed. She had a bad infection suspected to be contributing, but as the infection cleared her catatonia did not. Based on brain imaging there was also potential for a neurologic condition called normal pressure hydrocephalus, but a lumbar puncture had no effect.

She had no close family, but eventually her next door neighbor came to visit. He mentioned to me how the patient's dog had recently died before she became ill, and after that she just seemed despondent. That dog was everything to her by the neighbor's account. This raised my suspicion for catatonic depression, and so I consulted psychiatry. Psych meds were started but ineffective, so finally as a last ditch effort (she was becoming severely malnourished and rather than keep her alive artificially with a feeding tube the more humane thing seemed to be transitioning her to hospice) we transferred her to inpatient psychiatry (unfortunately she left my service for this reason so I didn't get to see her after this) and underwent a trial of several rounds of ECT (electroconvulsive therapy) for severe refractory depression. One of my co-residents happened to be on the psych wards at the time informed me that a few treatments in she was smiling, mobile, and conversant, and recalled me coming in to talk with her every day for nearly a month.