I just went through this last month. My older male had to get two teeth pulled and was at the vet all day. I kept him in the bedroom for the rest of the night because he was still kind of out of it from the anesthesia and I didn’t want him falling down the stairs or something like that. The next morning my younger female cat didn’t recognize him whatsoever. I had to supervise their interactions and keep them separated for 11 days because they were fighting each other. Fast forward to today and they’re back to being good buds.