It is important to see a dentist as soon as possible if you are unlucky enough to lose a filling or a crown. You may or may not have pain or sensitivity, but the longer you leave a tooth without a protective covering, the higher the chances of you possibly developing a problem, or exacerbating an existing problem. The loss of a filling or crown may also have occurred as a result of an underlying problem, for example dental decay.
If there is no decay, then your existing crown can normally be cemented back straight away. Lost fillings will usually require replacement of the filling, you may have the option of a temporary or definitive filling being placed.