Your question might concern what may have happened to Fortunato after Montresor walled him up in the niche and left him there. Naturally Fortunato would have died, but some writers have assumed that he died immediately of suffocation while others have assumed that he died of starvation. I believe it was Poe's intention to have the reader believe that Montresor's revenge included a long, lingering death for his victim. There was undoubtedly plenty of water, since the text specifies that there is water dripping everywhere and that they are in fact under a river. So poor Fortunato could have quenched his thirst by licking water off the rock wall to which he was chained. Poe also specifies that there is some air down there.
We passed through a range of low arches, descended, passed on, and descending again, arrived at a deep crypt, in which the foulness of the air caused our flambeaux rather to glow than flame.
Montresor also makes repeated references to the abundance of nitre. This substance contains a large quantity of oxygen. So it would seem that Fortunato could get water and some air in his confinement. The rough stone wall constructed by Montresor may contain enough chinks to allow the passage of some of the foul air from the other side. Montresor does not say so, but he might have left a few air holes in his wall.
So it would seem that Fortunato died of starvation while standing up. Perhaps in time his skeleton would have slipped through the chain and crumpled to the ground in the rags of his jester's costume. Montresor would have wanted his victim to suffer a long, lingering death. At the end, Montresor receives no answers from Fortunato, but that doesn't necessarily mean the man is already dead. That is unlikely. He has probably fainted or even refusing to answer.