side bar in twenty eleven

Sheila, deactivate all the plugins and try again.

if you want to have a page with a sidebar, attach the ‘Sidebar Template’ to the page:

dashboard – pages – edit page – page attributes (on the right, a bilt lower) – template

if you want to use posts and want a sidebar with the single post display, you need to use a plugin, for instance https://www.ads-software.com/extend/plugins/twenty-eleven-theme-extensions/, or edit the theme files.

before you edit any of the theme files, create a child theme to work with; https://codex.www.ads-software.com/Child_Themes