This happen when you have moved the PST file from its orignal location and replaced it again.
Click in Start > Control Panel > Mail
The Mail Setup dialog shall open up, click on Data Files button.
Now you may be confronted to 2 situations:
Path to .pst file is already present----> select the mail folder (highlight) and click on "Set as Default".
.pst file is absent -----> click on Add -----> browse for mail folder and click on "Set as Default".
Close the Mail Setup dialog.
Note that: Prior to these manipulation, make sure that your .pst file is not locked in read only mode.
Right-click on the file > Properties > Uncheck "Read-Only" attribute.