My left knee locks when i try to straighten it. what could be the cause?

See below: This could be a tear of your meniscus which is a little sliver of cartilage that is both on the inside and outside of your knee. If it gets torn, the torn piece may move when you bend the knee and then prevent it from straightening out. You should see an orthopedist about this.

See an orthopedist: As an orthopedic surgeon who does mostly arthroscopic surgery, you likely have a meniscal tear. If your knee is locking that is the mostly likely cause. You may have a loose body in the joint. Get a good ortho exam and possibly an mri.