A filling coming out is rarely painful at the moment it happens, which is why it is often postponed. What follows depends on what the filling was covering.
Exposed dentine is not an outer surface
Beneath enamel lies dentine, threaded with microscopic tubules running toward the pulp and containing fluid connected to nerve endings.
Enamel normally seals those tubules from the mouth. Once a filling is lost, the dentine floor of the cavity is directly exposed to temperature, pressure and sugar.
The resulting sensitivity is often the first sign, and it reflects fluid movement in the tubules rather than damage occurring at that moment.
The cavity shape works against the tooth
A prepared cavity has defined walls and a flat floor, a shape designed to retain a filling rather than to be self-cleansing.
Left open, it collects food and plaque in a form that a toothbrush cannot displace, and the environment inside becomes persistently acidic.
Decay therefore progresses faster in an open cavity than on an intact surface, and it does so on a floor already close to the pulp.
Remaining walls are more fragile than they appear
A filling contributes to how the tooth handles biting force by supporting the walls around it, particularly in larger restorations.
Without that support, unsupported cusps flex under load and can fracture, sometimes below the gum line, which changes the available treatment considerably.
Chewing on the opposite side is standard interim advice for this reason, though it addresses the risk rather than the cause.
Why fillings come out at all
Recurrent decay at the margin is the most frequent reason, as the seal between filling and tooth degrades and bacteria work beneath it.
Fracture of the filling material or of the tooth wall retaining it accounts for many others, particularly in large or old restorations.
What caused the loss matters, because a filling that came out because of decay underneath needs different treatment from one that simply fractured.
Temporary measures and their limits
Over-the-counter temporary materials exist to cover the cavity and reduce sensitivity, and they are intended to last days rather than weeks.
They do not remove decay, seal the tooth reliably or restore strength, so a tooth patched this way is being protected rather than treated.
Persistent pain, pain to heat, pain that wakes a person at night, or any swelling indicate the pulp is involved and require prompt dental assessment.