Once a tooth structure is gone, it's gone forever. There's no way to regrow the lost tooth structure. The only solution left is to stop further spread of infection to other parts of the tooth or even other teeth. The treatment varies depending on what stage the caries is in. Incipient caries or white spot can be stopped from progressing by adding a remineralising fluoride solution or gel which will prevent the start of caries and will make the tooth structure more resistant to decay. Stained and deep grooves on the chewing surfaces can be filled with Pit and fissure sealants. These flush out all food and prevents the formation of any cavities.
Unfortunately if the decay is progressed beyond the initial stage, there is no way to save the tooth structure. The infected part of enamel and dentin must be removed.
Decay that is present only in the first two layers of the teeth can be treated by removing the infected portion by a rotary or hand instrument, and by giving a filling material to replace the lost structure.
But if the decay spreads to the underlying pulp portion or the 3rd layer, it's very difficult to save the tooth from going for a root canal treatment.
During Root canal therapy, The entire pulp chamber should be cleaned and irrigated and then a material is is placed in the chamber. A core build up is required after the filing material is placed. Depending on the extent of damage of the tooth, a cap might be placed to protect the tooth structure from breaking.
Often in cases of large cavities which are close to the pulp, medicaments are applied and left for a few months with a temporary filling. This temporary filling or dressing is later replaced by permanent material.