Widgets page not displaying properly, non-functional

I’m using the WP Clear-Video theme, have been for a couple years with no problems. Today, my widgets page is no longer displaying properly. The problem is on the admin panel, the widgets page (under appearance) does not load or display properly. The available widgets bar loads with some of the widgets showing on the left side, but the actual sidebar showing what widgets are currently installed and running does not appear at all (usually on the right side). However, on the actual blog there are many widgets installed and running properly. But now I can not edit, add, or remove any widgets.
I also notice that if I click the screen options bar or the help bar in the top right of the screen, that nothing happens. I’ve changed nothing in the code (I don’t know how to do that!) and don’t recall changing anything else since the last time I edited my widgets just 2 or 3 days ago. Thank you for your help!