The painting had actually been stolen the morning before, but everyone had assumed that it has been taken down to be photographed by the museum's photographer. When the painting was still not back the next day, the theft was discovered.
For over two years, all the world wondered about the fate of the Mona Lisa. Finally, it was recovered when the thief, an Italian named Vincenzo Perugia, attempted to sell the painting to a gallery in Italy. Perugia had stolen the painting on behalf of another man, the mastermind Eduardo de Valfierno, who had promised to pay him handsomely for it. Instead, the mastermind had also hired a forger to create six unimpeachable copies of the painting. Once Perugia had stolen the painting, the mastermind took a ship to America, where he sold the six copies to six different, anonymous rich businessmen, all of whom were convinced they were secretly buying the one true painting.
(Interestingly enough, today is also the anniversary of last year's theft of The Scream by Munch, stolen at gunpoint from the Munch Museum in Oslo, Norway. Say what you will about Perugia, but his theft showed a lot more style.)
The Day They Stole the Mona Lisa by Seymour Reit (out of print, but available in many libraries)