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 helps. You can deactivate all plugins at once from the Plugins page by selecting and applying the Deactivate Bulk Action.
If it does help then re-enable the plugins one at a time to find the culprit or culprits.
If you do not currently have access to the wp-admin 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 them 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.