My theory is that you accidentally fast travelled out of the prison by loading last checkpoint while highlighting a mission not in the prison. However, I would've expected you to return to the prison the next time that you load up the save file from the title menu.
If you've already been to the prison on that playthrough, fast travel maps will allow you to fast travel to the prison. kotekzot's theory that you haven't retrieved the antidote sounds slightly plausible too. In any case, you can click play at the title menu, new game, pick the same character, choose the chapter closest to your progress, which sounds like chapter 16 which is the start of the prison, as desireable, and select your old character's data to play with. Don't worry, this creates a copy of the save file.