Are you using Firefox? If so, try it in another browser. If it works, then Firefox is the problem.
First, it you have the NoScript Firefox addon installed, make sure to allow it to run scripts on your site.
I also had a problem when I upgraded to the new version of WordPress with a 3.x version of Firefox. I couldn't expand certain menus, and some arrows didn't even appear. I upgraded to version 5 of Firefox, and everything started working normally. Unfortunately, many of my Firefox addons didn't work, so I downgraded Firefox back to a 3.x version, and the problems with WordPress remained fixed. I can't really say why. Maybe a newer version of certain files remained installed with the downgrade.