Office XP Loose user info by itself

A user had office XP installed on his machine. His outlook.pst is almost 700 MB or you can say a gig long. Today, when he open the outlook no mails and all of his stuff was gone. When looked at under local settings can see two folder outlook.pst andoutlook1.pst. So, he still have his old files but he also lost contects and other info how to restore it? Any suggestions?