A visit to the dentist is warranted. Looks like there is a weakness in the tooth and will need some type of filling. Look for why this occurred so it doesn't happen again or to other teeth. Having your bite checked along with an orthodontic consult would be beneficial.
Hi! Yes this is serious! It looks like a bad cavity. Please go to your nearest dentist. Thank you!
Sounds like you may have wear that has exposed Dentin, and this can be restored with composite resin bonding. I would suggest you get an orthodontic consultation to explore straightening your teeth so that your teeth don't wear unfavorably.
Douglas Jopling, DDS
Dallas, Texas area