I have the same problem. The launcher had just been updated, I played for a while (with all the mods still there) and stopped playing for a few minutes. When I opened the launcher again it said it had a new modpack to download, even though the last update had been just moments before. I downloaded the new modpack and all the mods were gone. All the folders related to mods are empty. I deleted everything, downloaded the client from the FTB site and started from the beginning, it didn't help. I'm running on Windows 7 64 bits, but I don't think that information will help much, it seems that FTB forgot to put the mods in the download they made available