Dear Ana,
please look at the issue MDL-39949, the similar problem was already reported. This looks like some other javascript code on the page that has an error and therefore everything else stops working.
As Gareth has said already, can you please open the console in your browser (usually by pressing button F12) and look for javascript errors there while the page is loading. If you see any, please comment on issue MDL-39949
Thanks
Marina