Broken tendon. The thigh muscle forms as a tendon as it nears its insertion in the knee. When the thigh muscles contract, it straightens out the knee. It is essential to normal walking. When that tendon ruptures (or breaks), it almost always needs to be surgically repaired.
Torn tendon. Ruptured tendon means torn tendon. The tear can be partial or complete. Have it evaluated by a professional and get the appropriate treatment.