In rare instances customers have reported that, after mounting an Offline database or connecting to a Production database, they were unable to see any of the mailboxes within the store/database. After review it became apparent that this was caused by either corrupt MAPI files or latent disk storage.
To correct this problem, you will need to take the following actions
1. First ensure the Database files are on fast local disk vs USB, UNC mapping etc
2. If already on fast disk Update MAPI Files with non-corrupt copies.