What should you do if you discover a surgical pin was left inside you after surgery?

First of all are you sure that the pin was not supposed to have been left there by design. Sometimes pins are left in bone fracture mends.

If you are sure that the pin was left there by mistake you need to get a lawyer/attorney and follow their advice on having the ploblem diagnosed and removed (or left if this is the correct option). If appropriate they will seek to get you compensation for the problem.