Depends on the exact error, Outlook version and configuration.
In general you start with verifying if the pst-file still exists and scan it
for errors with scanpst.exe.

Signature
Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
http://www.howto-outlook.com/
Outlook FAQ, HowTo, Downloads, Add-Ins and more
http://www.msoutlook.info/
Real World Questions, Real World Answers
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>I keep getting an error message saying Outlook cannot open my personal
>folder
> and because of that Outlook will not open at all. What can I do to
> rectify
> this?
You can go thru the mail applet in the control panel to see drive/path
information for the personal folder defined. If necessary, correct the
drive/path or define a new one and remove the old.
>I keep getting an error message saying Outlook cannot open my personal
>folder
> and because of that Outlook will not open at all. What can I do to
> rectify
> this?