Do i need to see a dentist immediately if i lost a crown?

Depends: Probably not tomorrow, unless you have a sharp or painful tooth, or if it is in the aesthetic zone (front teeth) that might bother you. But don't wait too long, damage to the tooth may result from chewing on it, etc. Thereby making a new crown more difficult or impossible. Plus your dentist must evaluate the tooth to see why you lost your crown.

Yes: The crown is the protection for your tooth, and without it you risk further decay or infection.

YES: If you are lucky, its just a glue failure( consider that good day- i never get upset over those), however more often decay or a fracture is involved,, sometimes requiring root canal or even extraction. So if you havent already contacted your dentist , do so

Temporary cement: If dentist is not available, you can use an over-the-counter temporary dental cement. Remember, temporary fix is just temporary. See your dentist ASAP.