If the knee always locked continues, it can most likely be a sign of inflammation inside the joint, which is caused by a severe tear or stretch in the anterior cruciate ligament or the well-known ACL. If this problem is not treated, it can cause destruction or severe damage to the meniscus. Currently, reduce physical activity in the knee area, such as stepping, and see a doctor for soft tissue imaging and further examination.
The possibility of surgery will be high for you.
Good luck