Are your teeth technically alive? This isn't such a straightforward question. Your teeth are actually made of different layers, and some are more alive than others. The hard, outermost layer of the tooth is its enamel. This is the strongest substance in your body; but when it's gone, it's gone. Damaged or decayed enamel cannot regenerate. Beneath the enamel is dentin, and this can regenerate itself to a (very) limited extent. But are these parts of the tooth considered to be living tissue?

Pulp Necrosis

Essentially, the tooth's only living tissue is the section with its own blood vessels, and this is the tooth's pulp (or nerve), which is found in the pulp chamber in the very middle of the tooth. The pulp is very much alive, and as such, it can die. Pulp necrosis (the death of dental pulp following trauma or an infection) is a dead tooth. The tooth may appear to be gray, and will further darken without repair. 

Infection or Accident

A dead tooth is generally preceded by an infection or accident. Had the pulp's damage been noted at the time, you would likely have received a root canal to remove the infected dental pulp. However, and though it's rare, pulp necrosis doesn't always present the tell-tale signs of a dental infection, and therefore may be painless and free of noticeable symptoms.

The Crisis Hasn't Passed

The change of color may in fact be the first warning sign of your dead tooth. Just because the tooth's pulp has already died it doesn't mean that the crisis has passed. There's also the 

 cosmetic problem of the tooth to consider. See your dentist as soon as they're able to arrange an appointment for you. Despite the fact that the pulp is dead (and quite possibly long dead) it still needs to be removed. The dental infection that led to your pulp necrosis could affect your other teeth and oral tissues. 

Removing Necrotic Tissues

A root canal may be recommended. The dead dental pulp will be removed from the tooth, and then your dentist will fill the pulp chamber with biocompatible dental latex before sealing it. The treated tooth will be inspected at a subsequent appointment, and although the removal of your necrotic pulp tissue may restore the tooth's color to its former vitality, this doesn't always happen automatically.

Concealment

If needed, your dentist can arrange a cosmetic solution to conceal this discoloration. It's most likely to be a dental veneer fitted to the outward-facing surface of the tooth or a dental crown fitted over the entire tooth. These restorations are made of robust porcelain and are intended to mimic the strength and appearance of natural tooth enamel.

Ultimately, you will want to see your dentist if one of your teeth starts to unexpectedly take on a darker color.