When I load TWD it says that my telltale account doesn't have access to this game, I have already beat the first season and downloaded the 400 Days pack a month or so ago and wanted to play but can't
TTG is working on a fix.
Meanwhile you should be able to work around it by deleting the Login data from the registry.
First, open a command window.
Windows XP: Go to your start menu and click on run. Type in "cmd" and press enter
Windows Vista / 7: Press the Windows key. Type in "cmd" and press enter
Windows 8: At the start screen, type in "cmd" and press enter
Then copy the command line below and paste it into the command window, press enter, and confirm the question:
REG DELETE "HKEY_CURRENT_USER\Software\Telltale Games\Launcher"