Mail shows number of unread messages, but no messages

The Apple Mail on my son's iMac 5, running OS X 10.6.8, shows the number of unread messages (9), but if I click on InBox, the messages themselves don't show up in the main window. This suddenly happened, and none of his old, read messages show up either. But he can access them just fine from his iPod, or through his Network Solutions web mail interface. Also, in Mail we can search part of his email address, and all the messages show up, read and unread! He has no rules set up. The connection doctor shows the connection is fine. What happened, and how can I fix it?