Sometimes there are plugin conflicts that can case fatal errors on your site and/or prevent users from logging into wp-admin.
This happened to me recently where a bug in a reCaptcha login plugin was preventing WC Shop managers from logging in. This bug was also preventing the admin, me, from authenticating as well.
In cases like these it is necessary to manually disable the plugin.
The easiest way is to disable them using a database editing tool like PhpMyAdmin. Note: use extreme caution when performing manual database edits on a live environment.
- Open the table wp_options
- Search for the key active_plugins
- If you have many plugins installed and only want to deactivate one, you can copy the value and then use this online Serialized Editor tool to remove the plugin from the list.
- Paste the result back into the value field and then save.
- Load your site to confirm there are no issues.