A meniscus tear
The most common cause of a locked knee is a large meniscus tear. When this happens, a portion of the meniscus gets stuck in a bad spot.
This doesn't allow the knee to move freely, causing it to lock. Displacement of the meniscus root is a certain kind of tear. Sometimes, it can be repaired which can be hard. See an experienced surgeon. An effusion is fluid in the knee, likely from the tear.