Sometimes profiles corrupt. You can try and use scanpst.exe to repair the affected pst file, it might work, it might not. Instructions are aplenty on the web, but this page looks alright:
http://www.emailaddressmanager.com/outlook/repair.html
What version of Outlook are you using? Outlook 2002 and earlier had limitations on PST sizes - 1.7gb. If your PST file grew larger than that, it'd usually crash. Microsoft would not guarantee that scanpst.exe would fix it. Outlook 2003 technically removes the 1.7GB limit, but it can still do some odd funky things from time to time. PST files created with Outlook 2007 have no limits, and are the most robust imho.
Dave