How can I access the space administration menu when the sidebar is hidden?

By adding the "hideelements=false" parameter to your URL you can temporarily disable HideElements.

This will make the sidebar show up again.

Example URLs:

https://your-confluence.com/display/SPACE/Awesome+Page?hideelements=false
https://your-confluence.com/display/SPACE/Awesome+Page?foo=bar&hideelements=false