It is usually very unexpected for people who use dental crowns to experience them coming off. When people see the cementing process of the dental crowns, they assume that the crowns are going to be there for life and that there2s no way they can ever come out. It is also due to this that they often do not learn what to do in such a case or take any precautions.
Infection is not the first thing on people's minds when their crowns come out as the area that is now exposed is not some soft tissue like gums or skin but the inside of the tooth on which they have had a dental crown.
This however, does not mean they can't be infected. Teeth are covered with a protective layer which prevents them from getting infections and keeps them very rigid at all times, but the same can't be said for the insides. The insides of teeth are vulnerable to infections, especially if food and other substances constantly touch the area. It is best to keep the area as clean as possible until you get a new crown or a replacement crown.
Insides of teeth are extremely sensitive, which means that if your crown falls out of its place, you are way more susceptible to any sort of pain that may come from eating solid foods or drinking liquids of high or low temperatures. The tooth also becomes structurally weaker, meaning it is more likely to chip or break if subjected to high force. The exposure of a tooth's insides for a long time can also cause tooth decay, so the situation is certainly an emergency and should be treated as quickly as possible. If the gums hadn't closed yet, in severe cases it might lead gum recession and gums not growing after dental crown operation.
Needless to say, seeing a dentist as soon as possible should be your foremost priority. Until you do see a dentist however, you should try to eat as little as possible and choose soft foods or medium temperature soups as your temporary diet. You should also try to keep the exposed tooth as clean as possible by using preferably some form of fluoride based mouthwash. There may be different consequences of the dental crown falling off that have not been listed, so be wary. You might also want to read do dental crowns cause bone loss.