Clicked on downloads in Steam and it said something like 'Missing Download Files, Update Required.' Hate to be the bearer of bad news, but if my problem sounds similar to yours, reinstalling should fix it. Because of this issue mods I needed to play LWOTC wouldn't show up in the launcher, no matter what I tried before the reinstall. Unfortunately the only thing that worked was reinstalling the game, verifying the files didn't work.
Had a similar problem a few days ago (on a desktop, Windows).