What happens to a tooth with a cavity that doesn't get a filling?

Serious stuff!: It will continue to decay, leaving less and less tooth structure available to repair or salvage it. You will likely need root canal therapy and\or a crown if you wait too long or if there is already extensive caries present. Even worse would be the need to have the tooth extracted if it is un-repairable. Eventually it will cause an infection that can spread to other parts of your body.

Toothache or worse: Certain bacteria in everyone's mouth digest carbohydrates and produce acids. These acids cause a loss of minerals from tooth, which softens them. This is decay. Left unchecked, these bacteria infect nerve in tooth, and can cause toothache. Tooth may break (enamel shell is brittle and depends on inner portion [dentin] for support). Pain, expensive repair, or tooth loss occurs if no treatment.