Meniscus tear? Did you twist the knee when you fell? Often missed on MRI's, can cause locking straight and buckling
Possibly a bone bruise to the knee cap or other bone in knee? (these are nasty and can take a year to heal) . MCL injuries can also take many months to heal if severe
Go see a sports physio with knee specialism (privately) or better still a knee specialist (if in UK, this will be an orthopaedic surgeon) - you'll need a referral from the GP (even if you decide to speed the process and go private)