Outlook Express uses a file type called dbx or mbx in some of the older versions. Outlook express is, in fact, different enough from Outlook that confusing the two could become problematic.
The erasure could have been caused by a virus, an overloaded mailbox or damaged dbx files.
Outlook Express Email Recovery repairs the dbx files themselves. Once repaired, you should have no issue dragging your emails back into place.
PLEASE NOTE: If you have Outlook files with a pst suffix, you need to use Outlook Mail Recovery.