Thousands of lost emails Outlook Express

I am providing support for a lawyers office on the side. One day he called stating that he had received a meesage when he opened OE, clicked on YES, something happened for a few seconds, and his emails dissapeared. He used to have 65,000 emails in his OE and now at the most he has 3,000 emails. He said that the message said something about compacting, or something as such. Does OE archive emails like Outlook and places them somethere else? I have looked everywhere but cannot find anything. Any ideas?