Tears dislocation. The structures that could be injured include cartilage, . Meniscus, ligament, tendons, etc even a fracture ca occur. You need an evaluation.
Many possibilities. Overextension can cause damage to tendon, cartilage, ligament, or bone. Have it evaluated by your orthopedist and get the appropriate treatment.
Swelling or scarring. Acutely after acl reconstruction the knee can be swollen and this could hinder knee extension. Physical therapy is started quickly after surgery to regain extension. It is vitally important to regain knee extension asap or scar tissue can form in the front of the knee. Once this scar tissue forms extension maybe very difficult to obtain and surgery my be required.
Swelling. The swelling following ACL surgery prevents you from fully extending the knee, but keep working on it in physical therapy. Hopefully you regain your full extension.
Limited knee ext. It depends upon the cause. Check with imaging to make sure it's not a fracture or loose body. If it it purely soft tissue outside the joint, then progressive exercises with pt, nsaids, modalities/laser may help.