Have you tried to reinstall using the Autoupdater?
Open Autoupdater / Commands / Reinstallation.
This will check your GTL install to see if you have all cars and tracks etc. It doesn't take long and it does not download everything again...
When it's finished it will list cars and tracks that are missing so you can install them.
If this doesn't work then I suggest that you try uninstalling & reinstalling GTL again. DO NOT use the windows system to uninstall it as this will remove your important user data etc, and as long as you don't physically remove files yourself it should be OK.
Please save your 'UserData' files first though just in case you make any mistakes.
Good luck and please feel free to use my picture on your mantlepiece to frighten the kids away from your fire place!
Cheers,
Gee