On some systems, entering this Dungeon in turn-based mode will generate an error message. If so, and if your game doesn't crash, you will be unable to have Simon Templar join your party. Even if you click "Yes" he will go to the Aventurer's Inn and wait for you. It appears that this error won't happen if your system allows you to hire roster PCs in turn-based mode.
Your safest choice is to start again from an earlier save. Many players of MM6 and 7 will avoid using an Autosaved game because of past bugs. Your Autosave may be safe, but you can never be totally sure.