locked knee

If you have sustained an injury to the knee that causes soft tissue, cartilage or bone to move abnormally within the knee this can cause a true locking of the knee. True locking is the inability to fully straighten out the knee and if left untreated for weeks or months can lead to a fixed flexion deformity. This differs from other symptoms of locking where the knee is held straight and is difficult to start to bend (usually due to a patellofemoral issue).

If the knee is truly locked then non surgical treatment is not likely to overcome this and key hole surgery is usually necessary. 
The most common causes of true locking are meniscal tears, tears of the anterior cruciate ligament and loose bodies.