Unless some natural or magical disaster stuck it, it should still be there.
Now, why it disappeared on the maps and such, I can only speculate. But if it was a big city under the Emerian Empire, and it was also the northernmost big city, it's safe to assume one or more of the following things happened :
- the city might have fallen on hard times and most of it's population went away ( like an almost ghost town ).
- the city was mostly a gateway for Emerian Empire goods, with the fall of said empire, most of the trade disappeared, so the people that lived of the trades left for somewhere else ( Sel-Kai ) where they could do their business.
- the city was ransacked in the turmoils of the fall of the Emerian Empire.
Some of the people came back, but others found another place safer ( in swamps... Sel-Kai )
- the city population didn't evolve much since the fall of the Emerian Empire, but cities like Sel-Kai and Lethys are really bigger and more important to the current Kulthea that it's not worth mentioning Corinn anymore.