What to do when you have cut your tendon on top of big toe?

Cut EHL tendon. If tendon on top of big toe (ehl tendon) is cut, one should get to the nearest E.R. Or foot specialist's office to get the great toe splinted in a p/op shoe or splint. Surgical repair of the cut tendon if done in a timely fashion will restore hallux dorsiflexion. If left unrepaired, the ends of the cut tendon would retract, making it difficult for subsequent repair without use of a tendon graft.
Tendon repair? See physical medicine doctor or orthopedic surgeon, you need early evaluation to see if tendons can be salvaged, repair or tendon transfer might be possible. If not bracing in your shoe should return your function to normal.