There are two general types of locked knees. The locked knee can either be caused by a mechanical block to knee motion, or a locked knee can be caused by pain that is too severe to allow knee motion.
When a locked knee is caused by a mechanical block to motion, there is something that is physically being caught within the mechanism of the knee. Often the cause of a locked knee in this situation is a "bucket handle" meniscus tear. When this type of meniscus tear occurs, a large fragment of the torn meniscus can become wedged within the knee, preventing normal movement of that knee.
Patients can also have a locked knee when they have severe pain with any knee motion. While it may be difficult for a patient to determine whether or not there is a physical block to their knee motion, or if pain is the issue, a good physical examination can usually separate these two types of problems.