I had arthroscopy surgery on my knee may last year and on tuesday morning i twisted my knee walking up the stairs and it locked up & is still locked?

Have knee evaluated: Then call your orthopedist and have that doctor reevaluate the knee. There could be another tear.

Possible retear: You may have a new tear in the meniscus (cartilage); or possibly a loose body. Sometimes this can flip between the femur and tibia and cause locking. You should see your orthopedist for proper evaluation and diagnosis.