Adding navigation menu to single post view in Twenty Eleven theme

the nav bar code (horizontal nav) is in header.php;
and if the horizontal nav does not show in single posts, thene your theme is seriously broken.

the sidebar does not show in single posts – if you need it there, try to work with a plugin https://www.ads-software.com/extend/plugins/twenty-eleven-theme-extensions/

I’ve intentionally removed the horizontal nav and added a plugin which creates a vertical nav bar

added a plugin which creates a vertical nav bar

ok – so your ‘problem’ is with the designed sidebar behaviour;

follow this suggestion:
– if you need a sidebar in single posts, try to work with a plugin https://www.ads-software.com/extend/plugins/twenty-eleven-theme-extensions/