Make sure that you have Show all topics selected at the top right drop down box. If after this setting has changed you are still convinced that posts have disappeared, you may have been hacked.
You should make sure you have the latest security fixes installed. Check the Announcements: community forum for the latest fixes, regardless of whether the Show all topics setting solves your problem or not.
You may also search these forums for 'hacked' or similar. You'll find plenty suggestions on what to do to recover and protect yourself.