If you are unable to completely straighten your leg, that is probably indicative of some floating cartilage(a piece of your meniscus) get caught between the ends of your femur and tibia. Your best bet would be to go and see an orthopedic surgeon, get the ACL reconstructed and have your medial meniscus scoped and the floating cartilage removed. Then take it easy for 4 months, doing post operative exercises to strengthen the muscles surrounding the joint capsule. Don't be too eager to get back on the bike until your leg feels considerably stronger and stable again otherwise you run the risk of doing more cartilage damage if your leg gives out badly again
Rgds Doug