Something went wrong and somehow you have wrecked your site. Argh!
No panic! Just go to Manage -> Site Maintenance -> Backup and Restore Database. Your database gets backed up automatically in the early hours of each morning, so each day you can always go back and restore yesterday’s data. Before you go any farther, though, it would probably be wise to backup your database as it stands right now, especially if you made some other changes before you noticed the problem.
If you are not sure about what has been corrupted or what needs to be done to fix it, it might be an idea to first restore an old backup to the Test Site so that you can check it all out and experiment with fixing it without risking any further harm to your data. Then once you are confident of what needs to be changed or restored, you can then repeat that on your live site. And don’t forget to make another backup after you are happy with your repairs.
To get to your Test Site, go to Manage -> Site Maintenance -> Go to Test Site.