I am unsure if this is related but I installed this plugin minutes ago (and did not activate it at all network or subsite...just installed in). See the last part of this thread of what the plugin author said to do but he said he fixed it etc. I did not change code manually at all. I just installed the plugin and did not activate. https://wordpress.org/support/topic/the-plugin-prevent-wordpress-seo-and-screen-options-from-working?replies=7
I just deleted the plugin from the network.
The admin area works fine. I have added support access for you as well in the dashboard.
Like I said, I am not sure if the plugin install is the cause (but considering the issues with it recently it could be...see above thread).
Lastly, here are the latest plugin updates that have take place the last 2 days (do you see issues or getting reports with any of yours etc)?
Let me know how we can get it back up asap please.