Word going around the Ubisoft forums and on another site has some Assassin's Creed II players getting stuck in "Hideout," a sequence about 11 chapters into the game, with no way to get out or advance the game.
The glitch appears to be triggered if one turns off the console right after the autosave at the end of Chapter 11. The game then loads up the hideout the next time the game is started, but nothing else loads from the corrupted savefile. Key features of the environment (I won't say which, for spoiler purposes) aren't available so there's no way to advance. You're just running around in a confined space, no way out.
An Ubisoft community developer acknowledged that the ACII team is aware of the problem and "are doing our best to fix this issue as fast as possible." Unfortunately, it doesn't appear a bug fix has rolled yet.
Reports don't say if this affects all versions or just one console, and if the latter, which one is even affected. I have not replicated this glitch - because I don't have the game. The best bet is to just manually save the game after Chapter 11 as soon as the opportunity presents itself. If you have a busted gamesave, your only option is to start over.