I just finished upgrading to Craft 3 RC and all text on the site is gone. The WYSIWYG Editor has been replaced with this message:
Unable to find component class 'craft\fields\RichText'.
With RC1 the rich text editor was moved to a plugin. You can find the Redactor editor (the previous richtext field) in the plugin store along with a new CKEditor editor as well.
You can read about this in the docs, too: https://docs.craftcms.com/v3/changes-in-craft-3.html#rich-text-fields