What should i do if i can't extend my leg?

Extension loss: If you can't extend (get leg straight) thats a real problem, if there was trauma and the muscles are working you may have a tear of the quadriceps tendon or patellar tendon or even a patella fracture. If it was due to a lower demand sports issue you may have a meniscus tear or even a "loose body" where a piece of cartilage and bone break off and block the leg. See an orthopedic surgeon.