You may need to reinstall the locally installed files that Minecraft puts into place.
To do so, start Minecraft, press 'Options' (above the 'Login' button), Press 'Force update!', then press 'Done', and login to Minecraft. See http://i.imgur.com/5b09B.png for visual instructions.
To do so, start Minecraft, press 'Options' (above the 'Login' button), Press 'Force update!', then press 'Done', and login to Minecraft. See http://i.imgur.com/5b09B.png for visual instructions.
Stu