I'm assuming you had a crown that came off. Not important to keep it hydrated with water or milk, that is done when a tooth has been knocked out and the dentist will be reimplanting it within a couple hours. With your crown, just put it in a box so that you don't lose it or break it and see your dentist as soon as can be arranged. Usually this is not an emergency or reason to get in immediately. Stay away from cold or acidic foods which can make the uncovered tooth uncomfortable. If the tooth has had a root canal, then there is no nerve and just don't eat on it, as teeth with root canals are weaker and may break if there is not a crown on it.