Haydn's portrait, painted in 1792.
Joseph Haydn is one of the most prolific composers of the Classical Era. With well over 100 symphonies, several concertos written for various instruments, and a few operas to his name, the library of his work has proven quite difficult to keep accounted for. He was a friend of Mozart's and a teacher of Beethoven. He continued to work well into his old age and up to his death garnering the nickname "Vater Haydn" or in English "Papa Haydn".
In 1979, Esterhaza, the opera house of which he worked was set on fire and many of his non-widely published works were destroyed. Every once in a great while, a work is discovered in a score copy, but many are feared to be forever lost. While his operas and symphonies remain mostly intact, a vast majority of his string quartets, concertos, and piano pieces are yet to be found. His most famous concerto, the one for Double Bass in D Minor only has 2 measures surviving.
Another piece of (now formerly) missing Haydn-related material was a piece of Haydn himself. Shortly after he died, his head was chopped off and stolen by grave robbers. A decoy head was put in its place. The skull wasn't found until 1954, when it was finally reunited with its body. Haydn is now one of the few individuals whose grave has 2 skulls in it.