My suggestions (be careful!):
1. If you have games in progress, either make a backup of the "saves" folders or just make a backup copy of your entire FTB folder and put it in a safe location.
2. Delete the original FTB folder.
3. Download the FTB launcher and run it.
4. Now you have a fresh, new, good installation of FTB, and you can download the modpacks that you were playing. Once they are downloaded, exit the game and copy your "saves" folders from the backup you made in Step#1 to the new FTB install location.
I hope you understand this. Be careful, do not delete anything if you do not understand! You must copy your existing worlds before you do this, otherwise you will lose them. But when you copy them back into FTB at the end, you will still keep your old games.
It is much easier if you do not care to save your old games, then just delete FTB folder, download it and re-install it again, and that should fix your problem.
