The most common reason for this issue is a corrupted SiteBuilder database. The possible solutions are:

1. To restore your database. If you don't have your own backup, you can order a Database restore from your Customer Area > Exclusive Professional Services > Back-up restore > Database Restore

2. Rebuild your site from scratch. In this case you have to delete and recreate the database sitemagixprod. You check this tutorial for more information on how to recreate the database.