According to some upset posters over on the official LittleBigPlanet forums, the Metal Gear Solid DLC released for the game last week didn't just bring good times. It brought bad, content-erasing times as well.
Some users are complaining that, after purchasing the Metal Gear content, their singleplayer progress through the pack was erased. Sucky, but surmountable. Other users, however, are complaining of having their user-generated content erased by the DLC. That's...less surmountable.
This being the silly season, Media Molecule are off being silly like the rest of us, so a comment/solution/acknowledgement of this issue probably won't be with us for a day or two.
LittleBigWorkshop : The Pod : General Discussion : Metal Gear Solid Level Pack Glitch [LBP, via PS3F]