I just looked it up…
The Mona Lisa was stolen once, way back in 1911. It was missing for two years. The man who stole it tried to sell it to an art dealer in Italy. I guess he thought that no one would notice which painting he was trying to sell.
Hopefully it was. The Mona Lisa is currently valued at between $800 million and $1 billion. It’d probably be valued at $0 if you were able to steal it and try to sell it