Extension lag is caused due to weakness of the Quadriceps group of muscle especially the "Vastus medialis Obliqus" most commonly(Unless the patella has been touched or Quadriceps muscle has been severed).
So the treatment is mainly aimed at strengthening the Quadriceps to regain that terminal extension.
Next Steps
Thorough Quadriceps strengthening exercises will definitely help
Health Tips
Online consultation will help rule out any other causes and to assess if permanent