If the Personal Folders file (.pst) or Offline Folder file (.ost) will not open or if either is suspected to be corrupt, refer to the following applicable article for repair procedures:

Outlook 2010

http://office.microsoft.com/en-us/outlook-help/repair-outlook-data-files-pst-and-ost-HA010354964.aspx

Outlook 2013/2016

http://office.microsoft.com/en-us/outlook-help/repair-outlook-data-files-pst-and-ost-HA102749454.aspx