There is no film that has won the Oscar for Best Picture and then disappeared.
There is, however, one Best Picture Oscar winner that has partially disappeared. This is The Broadway Melody (1929).
It was the first talkie to win the Academy Award for Best Picture.
This musical had the particularity of containing a sequence filmed in Technicolor, while the rest of the film was in black and white. At the time, this surprise caused a sensation.
However, this color sequence has been lost. Only a black and white copy exists and appears in the version of the film that has come down to us.
If no winner of the Oscar for Best Film has ever completely disappeared, we can however point out that several films nominated for the Oscars have completely disappeared from circulation, as well as certain winners in various secondary categories.
Until the 1950s, studios did not take much care of their archives and it was common for a film to be lost or destroyed without anyone realizing it.