Do you mean that you had a separate About page that disappeared? Or do you mean the default theme had a small About profile in the sidebar which was eliminated when you equipped your sidebar with widgets?
I'm not sure. I think what I had in the sidebar was the word "About" that formed a link to a page that contained the About text.
If it's yes to the second question then that's how the theme works. You'll have to create either an About page or a text widget that functions the same way to achieve the same end.
I assumed using widgets had eliminated the theme's sidebar, I just thought About was such a ubiquitous feature there would be a widget for it.