My b2evolution Version: 2.x
I was in the middle of configuring my Sidebar widgets and I started by removing all of them.
When I got done removing all the widgets from the "Sidebar" section, the section was gone. This is in the backoffice Blog Settings | Widgets Tab.
Is there a way to get it back?
If I select a different Skin, I get the section back but when I switch to my old skin, the Sidebar section is not there :-(
I think what you need to do is "reload" the skin on your global settings tab.
Or maybe try my favorite piece of voodoo: on tools->misc delete your pre-rendered cache.