If you go to your mail folder under docu&settings-user-local settings-application data-microsoft-outlook express, you will see inbox.bak or some backup file for your inbox. make a copy of the file and rename it to inbox.dbx (I am guessing this name, look at other extensions).
Make sure you change windows settings to show hidden files.
To restore mail folder of Outlook Express those are unable to open right now, you have to use effectual DBX Recovery Software. I recommend you to run Outlook Express Restore software in your PC surely you can restore disappeared folders and easily can open DBX file within 1 minute.
Simple solution of your query available here-