This problem just started today. It was not there yesterday. I have made no site changes since yesterday.
I am using your basic 20-10 theme on my WP Multisite. All themes and plugins are up-to-date. The same problem exists on all my sub-sites.
To see the problem, go to http://polymontana.com
You will see the home page giving a short list of the post summaries and a blank right column where the widgets are supposed to be. I say "short" because it normally shows a lot more post summaries.
Then click on any post title. You will see all the widgets properly displayed for the individual post.
Even though I can login on the single post display, when I return to the home display, my WP bar at the top is gone.
Any ideas what is causing this?