|ost files are cached Exchange files. By default, they are stored in the C:\Windows\Users\userid profile folder.|
How are you trying to access the file? Are you using the same computer you used with the old account?
If so, login with that old username (should be a cached login). Open Outlook and export into pst format whatever you want from the mailbox.
If all else fails, ask your Exchange Administrator to export the information from your "Old" Exchange mailbox.