It is Outlook.ost that is the offline Outlook db, I believe.
Is your outlook running and in offline mode?

Maybe the file is corrupt? Try this:

Close Outlook.
On the Start menu, click Run.
In the Open box, type

drive:\Program Files\Common Files\System\Mapi\LocaleID\scanost.exe

where LocaleID is the locale identifier (LCID) for your installation of Microsoft Office. For example, the LCID for English (United States) is 1033. For a list of LCIDs, see Microsoft Office Help.

Click OK.
Click either Scan all folders or Choose folder to scan. If you click Scan all folders, this may take several minutes.
Select the Repair errors check box.
Click Begin Scan.

Details of actions performed during the scan and repair are saved as a message in your Outlook Deleted Items folder with the subject “OST Integrity Check.”

If the Offline Folder file cannot be repaired, you can disable it and create a new one.