2.1 If your issue is a CTD
(crash to desktop) or unexpected error open your SS.System.log file in \Medieval II Total War\logs\ with Word, OO or notepad and include the last 10 lines (6.3 needs 100 lines or better yet the full log as zipped attachment) in your post. In case you are posting more then 10 lines be sure to use a spoiler tag

enclosing the log (use [ spoiler ] contents of system.log [ /spoiler ] without the spaces). 6.3 and 6.4 users should access their log from start menu -> stainless steel -> error log.
2.2 If your game freezes or crashes on an AI turn tell us the last faction active (as indicated by the highlighted shield/heraldry at the top of your screen) before the crash. Use "follow ai movement" and the reveal map cheat "toggle_fow" to identify the last action of the AI before the crash. Often campaigns can be saved by simply preventing that action.