Why this happens

This happens when PHP code calls a function that was never defined - usually because the plugin or theme that defines it isn't active, wasn't fully installed, or was replaced by a newer version that renamed or removed that function while another part of your site still expects it to exist.

Step-by-step fix

  1. Check the error message for the file name mentioned (e.g. /wp-content/plugins/xyz/file.php).
  2. Rename that plugin's folder in File Manager (e.g. xyz to xyz-old) to temporarily disable it.
  3. Refresh your site and check if the error is gone.
  4. If it is, reinstall the plugin with a fresh copy from WordPress.org and delete the old one.
  5. If the error relates to a theme file, switch to a default theme (Twenty Twenty-Four) under Appearance > Themes.

How to prevent it happening again