@ARE_you_kidding_me used the same references I was going to use.
Many great works have been stolen, cached somewhere and lost to public viewing. Whether or not they have been destroyed in the ensuing wars, by accident, by fire as @Jeruba mentions above, is anybody’s guess.
You might say that Venus de Milo was destroyed. She has no arms. I’m sure that wasn’t Alexandros of Antioch’s original intention. There was an Italian guy who was arrested by the Nazis during WWII for trying to sell her right elbow to Herrmann Goering. LOL. It didn’t end well for him.
Many works have been stolen and forged so many times that the museums really aren’t sure whether or not they possess the true originals. Especially when it comes to van Gogh. He’s the most forged artist in history.
The theft of the Mona Lisa in 1911 is one many aren’t sure of. And a great story of forgery. It is said the Louvre’s basement is full of her forgeries.