Megaupload i'm pretty sure has something in their end user license agreement that says "if we lose your data, we're not liable" (in case megaupload for some reason isn't able to pay for the servers their leasing). Megaupload is great at paying their bills, but obviously right now they're not. I'm not saying that the end user license agreement of your average popular cyber locker is going to hold up in court. I'm saying that possible loss of data is a known risk of using any cyber locker service. Anyone who thinks other wise isn't thinking about the leased server part too much.
Saying that the fbi deleted data wouldn't fly either unless they actually did. Suing because the fbi put your data in a situation to be deleted would be more accurate. But, because of the known possible risk of using cyber lockers, the suit may go no where. And lawyers will be big time ass holes in court about this if someone were smart or dumb enough to sue over (i'm thinking dumb enough, doesn't seem like the best thing to sue over unless there was a guarantee of data safety which there usually isn't with cyber locker services just because of the way they do business which lets them have storage capacity on hand for people to use). The way megaupload does business to have server storage capacity on hand for free public use is why they don't have a guarantee of data safety in their end user license agreement.