Outlook Express DBX files are quite fragile and it must have gotten corrupted from being large or not maintained. First, close OE and make a backup of ALL the filess in the data store folder. Copy everything for safe keeping.
Something like this path:
C:\Documents and Settings\UserName\Application Data\Identities\{A85341F9-0683-40CE-AE30-FB7311AF7D3B}
In the OE data store folder, rename the folder in question from xyz.bak to xyz.dbx and then open OE and hope for the best.
If that doesn't work, delete everything in the OE data store folder and copy the backup back into that folder. Open OE and create a new folder with the same name as the one that has vanished. Copy the .BAK file somewhere like the desktop. Rename the BAK file to DBX. Then do an Import in OE using the folder on the desktop as the source file.
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At this point in time you shouldn't even be using Outlook Express. It's very outdated, not being updated and it DOES have issues with being fragile. Also, apparently you have no backups of client email!! This is not a good scenario for a business environment. Think about Thunderbird if you want something similar looking to OE and also free.