Thanks for your reply. That's exactly what's been happening to me too.
I had a chat with Squarespace help and they asked me to take out everything in my Custom CSS box. After clearing my cache, the editing tools miraculously returned! Apparently I need to troubleshoot every line in my Custom CSS to see which one is the culprit. I haven't had the time to do this yet though.
So apparently sometimes when SS is updated, any custom code might break the editing pages. Crazy, eh?