In some cases a dead tooth can be saved depending on how bad it is and how soon the treatment for the dead tooth is done.
The longer you wait the less chance you have at saving the dead tooth.
The quicker you get treatment for the dead tooth the better chance you have at saving a dead tooth.
A dead tooth can almost always be saved by treatment which is done using endodontics which is root canal therapy.
The dentist can perform root canal therapy can reclaim teeth after they have died as the most common cause of a dead tooth is physical trauma, cracked tooth or even a deep cavity in the tooth.
If you wait too long to get the root canal therapy for the dead tooth then it may not be able to be saved.