What did the narrator do to Fortunato, and how long ago did this happen?
The narrator of "The Cask of Amontiallado" is Montresor. I believe you have confused him with Fortunato, the man Montresor kills. Montresor is telling the story of how he killed Fortunato fifty years before to an unknown confidant, perhaps a priest. Basically, Montresor lures Fortunato to his death by telling the latter he had bought a wine called "Amontillado". Fortunato considers himself an expert in wine, so Montresor asks Fortunato to look at the wine to verify it is indeed an Amontillado. This is just a ruse to get Fortunato down to the catacombs beneath Montresor's house where there is a room where Montresor can chain Fortunato to the wall. Montresor encloses Fortunato in the room, all the while taunting him. During the whole story, Montresor maintains that he is not insane.