Deciding not to undergo treatment for a dental problem is always an option, although it may not be in your best interest.
If you have a badly broken down, decayed, chipped, or cracked tooth in need of a crown, and decide not to have it fixed, the tooth can break apart further, to the point where it may not be possible to save it.
A crack may spread through the tooth into the "pulp", the hollow inside of the tooth where the blood vessels and nerve are located. Bacteria can get into the pulp, causing an infection that may require endodontic (root canal) treatment. In rare cases, the tooth may not be salvageable. If the tooth is lost, problems associated with missing teeth may occur. These include movement of the adjacent teeth, a change in the way your teeth fit together, increased forces on the remaining teeth, a reduction in lower face height, shortening of the chewing muscles (which can result in greater muscle forces), increased stress on the jaw joints, and the potential for jaw joint problems (temporomandibular joint dysfunction, or "TMD").
Learn more: Diagnoses › Acute apical abscess
Learn more: Diagnoses › Loss of vertical dimension of occlusion
Learn more: Diagnoses › Temporomandibular joint dysfunction