As answered by one of the dentists on our Facebook forum.
A cavity once started is generally an irreversible process and requires removal of the decayed part of tooth and replacing it with an appropriate material. This procedure is known as a dental filling. However, if you brush with a fluoride toothpaste, the progress of the decay (caries) can be slowed down. The sooner you get your filling done, the better it would be. Caries will keep on progressing towards the centre of your tooth where the living tissue is present. And once it reaches the living tissue, you will have to get a root canal treatment done along with a crown which would be much expensive and technical than a filling. So buckle up and get your filling done ASAP. No need to wait for any pain or other symptoms to take place