My header bar has disappeared

I think you need to check if the site banner widget is available in the header section 

I'm not sure what you mean. Where do I check if the site banner widget is available?

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.

Which section of the Admin is this?

This is not in administration. It's in page management. (Pencil Icon) > Manage (Site|Group|Application) Theme > Edit this Page

The widget was removed. I have added it back.