ACL is the ligament which maintains alignment between the thigh bone and leg bone at the knee joint. If it is torn, in the acute phase one will have pain and selling in the knee and hence, find it difficult to stand and walk. In the chronic phase, there is usually no problem in standing, but there may be sense of instability while walking or running.
ACL tear needs to be repaired if it interferes with your daily lifestyle, eg. if you were running before the injury and now find it difficult to do so, or if there is persistent instability in the knee which interferes with your daily activities.
Next Steps
Consult an Orthopaedic Sports Surgeon who performs arthroscopy