I have noticed on a site that I have logged into recently that alot of the icons are missing from throughout the site, instead there are symbols in the place of the icons. I'm noticing it on the administrator login page along with any other backend page. The front end seems fine, just on the backend.
You are missing IcoMoon font or files are corrupted. Check into
media/jui/fonts folder and see if IcoMoon files exist. If not, you can download them from Joomla! official github repository in here. Just upload those files to
media/jui/fonts and it should fix that.
Maybe someone has accidently deleted font files. But it would be good to just override all Joomla! core files to be sure nothing more is missing. You can download your specific version from here. Then upload all files and directories (except
installation directory) to root of Joomla!.
Check your htaccess rules, and match protocols - they can sometimes trigger cross origin block.