IMAP, POP3 or Exchange Server mailbox?
If IMAP you may have exceeded the server size limit
If POP maybe you exceeded the max file size (depends on Windows version)
Outlook does have an inbox repair tool -
How to use the Inbox Repair Tools to recover e-mail messages in Outlook 2002, Outlook 2003, and Outlook 2007
How to find and run the Inbox Repair tool in Outlook