It IS possible that the file(s) in question were unable to be reconstructed by fsck that they were deleted. The fsck program only attempts to repair and tries very hard to reconstruct files as well as it can. However, it is by no means any type of backup. Any action that is performed by fsck is basically not reversible.
I'd be really careful about attempting to use portions of a MySQL database contained in a lost+found directory as a database is not contained in one file but many files which must be in "sync" to have any hope of database recovery in a mode where there is any type of reliability or data integrity.
As for file recovery, sorry, you'll have to go back to the backups that you have presumably been making since the data was important. Otherwise, you are really out of luck.
If the data is so important, then you MIGHT be able to try to enlist the help of one of the many data recovery services. It is pricey and the results are less than perfect.