I have had this problem twice now and it seems the only way to fix it is to reinstall your game client. However it takes me 5 hours to download the patch and i haven't been able to find a recent version of the game for download. (to use as a stepping off point and reduce the size of the patch required)

I have however found a method of short cutting the solution as follows:

Before you start, run the launcher and backup your settings (better safe than sorry)

1. locate your client install directory (for example c:/games/squareenix/final fantasy xiv - a realm reborn/)
2. rename your client install directory to something like "backup"
3. run the installer and UNINSTALL the client
4. run the installer and INSTALL the client again (it helps if you install it in the same location you uninstalled it from)
5. locate your new install directory
6. rename the new install directory to something like "new install" (take note of the folder name before renaming as your about to use it)
7. move the old install directory (now called "backup" or whatever u called it) to the same location (unless you used the same location when you reinstalled)
8. rename the "backup" directory with the name from step 6.
9. run the game and everything should be copacetic.

Let me know if this works for you