WordPress – fatal error wp-widget.php

Solution:

There are many possibilities :

1. You update your wordpress site and one of your plugins is calling a deprecated file. If so, try to disable all of your plugins. (Obviously here, this file is not deprecated, but it is a general thing to do).
2. The site directories don’t have correct permissions. If so, do a chmod -Rf 755 on the site root.
3. Your installation is corrupted and you should reinstall WordPress.