You may have had a problem with an update or you may have a corrupted minified JS file.
The first step I would take is to download ACF from your ACF account and try reinstalling. ACF can be deactivated and deleted and data will remain. Then install and activate the fresh copy.
If that does not work I would test for the possibility of a corrupted JS file. This can happen to very large minified files. You can test this by adding define('SCRIPT_DEBUG', true); to your wp-config.php file. This will force the site to load non-minified versions of all script files where they are available.
