There are a number of variables that determine whether this is an urgent situation or not. For example, if the braces had recently been removed, there is a greater chance that the teeth will shift and move if a retainer is not replaced relatively quickly. However, every patient may differ in the potential amount of relapse. If a patient has been out of braces for a long period of time, and the teeth are in a relatively stable position, the chance of significant shifting may be low. Please call our office if your retainer is lost or broken to discuss your options.