The bar that contains the logo, search bar, and join/login buttons has disappeared and is nowhere to be found.
Is the Site Banner widget still on your page? By default, it's a locked widget (so that it inherits) in the header of the site theme. It's possible you may have unlocked it so it doesn't inherit, or removed it altogether. Either way, this is revertible through analyzing and editing the site home page's header section in page configuration.
Here is the Site Banner widget, locked by default, in the site theme's header region.
