Tabs and menu items not working in back end

A couple of weeks ago we noticed an issue where in the back end (v2.1.0) we could no longer access menu items from drop downs, nor could we when editing an entry use the tabs. We also cannot use the image selector when adding an entry. I’m assuming this is a javascript issue, but am confused because the site was working fine, then suddenly stopped behaving itself. Has anyone ever come across this before? The site is www.onekind.org - an animal charity.

Thanks, John