It is always recommended to make a backup of the site after making any change.
The first step in troubleshooting almost any issue will be checking for a conflict.
How to Check For a Plugin Conflict
To check for a plugin conflict, deactivate all plugins to see if it reolves the issue. If it does, a plugin is causing the issue. You can deactivate all plugins at once from the Plugins page by selecting and applying the Deactivate Bulk Action.
Once it is clear that a plugin is causing the issue, re-enable the plugins one at a time and check the site after each plugin activation, to find the culprit or culprits.
If you do not currently have access to the wp-admin page you can manually disable plugins using an FTP client by renaming the plugin. You can simply add a .bak to the end of it.
How to check for a Theme Conflict
From the WordPress Dashboard navigate to Appearance -> Themes and switch to a WordPress Theme.
If you do not have access to the wp-admin you can use an FTP client and then navigate to the wp-content/themes directory and download a backup of your current theme to your desktop. Now you can remove every theme besides a WordPress theme. If the themes directory only has a WordPress theme the site will activate it for you.