It depends. If it’s not hurting, then I wouldn’t consider it an emergency, but the sooner you can get it placed back on, the better.
Crowns not only act as protection for the tooth, but they also act as a space maintainer. Teeth in general like to touch other teeth. If your crown is off for an extended period of time (a few days even), the tooth under it may shift. Usually it extrudes, and tips forward in the mouth. Also the gums may grow up onto the tooth a bit.
These changes can make placing the crown back on the tooth a challenge. Often the crown has to be adjusted, or “skinnied up” on the side it tipped towards to get it to go back down on the tooth. This can open up spaces on the other side of the crown, causing food impaction down the road.
All this means that the quicker you can see your dentist to have it recemented on, the easier the process will be and the better things will be long term.
This is all assuming that the tooth underneath the crown was in good shape. If there is decay, then a new crown will be needed.