Most of the problems I've found that involve people not being able to complete the download are caused by the slow internet connection. This, to my understanding, is being worked on.
Everyone else. Save your saves. Try to do a clean reinstall of the launcher.
I recommend always saving your ENTIRE folder (where you put your launcher and all of the files it generates) onto a flash drive before ever updating.
But by providing such information, you are aiding people who are pirating minecraft. The FTB team does not support pirates.