I know several reasons causing this problem:
1. Outlook Express settings. Run OE and Click View | Current View and be sure it is set to "Show All Messages". Also verify that "Show replies to my messages" is not checked or you will see only replies to your messages.
2. Antivirus software. Sometimes happens Outlook Express *.dbx files (files where OE stores messages) are deleted by antivirus software. For example older versions of McAfee VirusScan are known to delete DBXs. If this happened I'm sorry to say your messages are lost.
3. Damaged *.dbx file. In the case of a damaged "Inbox.dbx" file, a new Inbox will have been created automatically, and will now be using the file "Inbox(1).dbx" or similar. However the original Inbox.dbx will still exist. Locate it by going to OE store folder (Tools | Options | Maintenance). Then you can try to extract messages from this file using DBXtract
Also, I recommend to backup your OE messages periodically to prevent such situations. I'm using outlook express backup
from abf-soft. Simple and handy (not an advertising, just sharing the experience).