In order to fix the issue, download and install the latest version of .Net Framework and run a registry cleaner like CCleaner or Glary Utilities. Also, clear your game cache, following the steps below:
Windows XP: Navigate to C:\Documents and Settings\"username"\My Documents\Electronic Arts\The Sims Medieval
Windows Vista or 7: Navigate to C:\Users\"username"\Documents\Electronic Arts\The Sims Medieval
Mac: Navigate to \Users\"username"\Documents\Electronic Arts\The Sims Medieval
Locate all the files ending in .package and delete them. After that, delete all data from WorldCaches and Thumbnails folders. If the problem is not solved, reinstall the game.