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I'm going to assume you're on Windows, since that's all I know how to fix
In the launcher, find out what the "Install folder" under the Options tab is and write it down somewhere.
Press windows key+r, type "%appdata%" in the box that appears and hit enter or click ok.
Back up the folder called "ftblauncher" to somewhere like your desktop (so it won't get lost), then delete it.
Now run the launcher. It's going to assume this is the first time you've run it since you just deleted its data folder. Just set the install folder back to what it was before (you wrote it down in step 1).
Hopefully that worked, if it didn't, let me know and I'll link you to some other instructions.