[Skunkworks] Outlook

Over the xmas holidays ane of my clients machine had a corrupted OS. I advised the client to get a tech where he was and reinstall another Os without formatting the harddisk. which worked out fine. After bringing the laptop back, the outlook pst folder has mails upto august yet the client was using the account even in December before the OS crashed. How do I recover the lost mails? how is this possible.?