I think that you have an error occurring in the server and debugging is off. So nothing is showing in the browser.
I suggest that you turn on debugging temporarily and see if an error message shows there. Its likely such a change would not occur for no reason. So think about what has happened on your site recently. A Moodle or plugin upgrade perhaps?
A common problem these days is Moodle installing the plugins with permissions that your server doesn't like.
Maybe check the poodll filter directory has permissions 755 for folders and 644 for files.