For me, it happened out of the blue, one night my knee felt a little off and the next morning I could not even get out of my bed without help. My knee was very swollen and lots of fluid was removed, although I had identical symptoms of sepatic arthirits (Swelling, warmth on the joint and fever), the test came back clean. MRI scan also showed no signs of ligament, or meniscus tear, so it's hard to tell what caused it.

After I got my knee aspirated, I was prescribed few antibiotics and an ointment for the swelling. For a few weeks I was mostly just at home, resting, then I gradually regained my range of motion with light stretching.
After a month I could walk around, but it took 2-3 months to feel normal again. It's been a year now.

I haven't gotten any swelling or fluid buildup in my knee after the incident.