Why this happens
Crowns and fillings can loosen over time, or come out after biting something hard or sticky. The exposed tooth underneath is often sensitive to air, temperature and pressure until it's protected again.
A safe temporary fix
Pharmacies sell temporary dental cement (sometimes labeled as temporary filling material) that can protect the tooth for a short time. If you still have the crown, you can sometimes reseat it lightly with a small dab of this cement — never with household glue. Avoid chewing on that side and keep the area clean by rinsing gently.
How soon you actually need to be seen
If there's no pain, a visit within a few days is usually fine. If the exposed tooth is painful, sensitive to hot and cold, or has a sharp edge, moving that up to a same-day or next-day visit is worth it — both for comfort and to avoid further damage to the tooth.
What the dentist will do
In many cases, an existing crown can simply be recemented. If the crown or filling is damaged, a new one will need to be made, sometimes requiring a temporary while the permanent piece is prepared.