Pulp necrosis or a tooth dying is a condition of the tooth where the nerve is dead in the dental pulp. The dental pulp is the soft tissue that contains nerves, blood vessels, and connective tissue inside a tooth. If the dental pulp becomes damaged or infected, it can die and cause a range of symptoms, including:

1. Pain: Initially, the tooth may be sensitive to hot or cold temperatures, but as the nerve dies, the pain may become constant and more severe.

2. Discoloration: The tooth may become discolored or appear gray or black.

3. Swelling: The gums around the affected tooth may become swollen or tender.

4. Bad breath or taste: An infection in the dental pulp can cause a bad taste in the mouth or bad breath.

If you suspect that you have a dead nerve in a tooth, it is important to see a dentist as soon as possible. Treatment for pulp necrosis typically involves a root canal procedure, which involves removing the damaged or infected pulp and filling the space with a dental filling material. If left untreated, pulp necrosis can lead to abscesses, bone loss, infection and other complications, including loss of the tooth.