The ACL is normally damaged following a twisted knee injury during sports or a fall. You can also damage it just by stretching your knee too far. Normally you’ll hear or feel a pop in the knee and the joint will then give way. You’ll feel a sudden pain and the knee will usually begin to swell in a few hours.
You might not be able to bear weight on the affected leg, and could feel unstable either when playing sport or, if the injury is more serious, during normal day-to-day activities.