how to troubleshoot and recover your broken wordpress site » wordpress consultant: judith kallos

The steps to troubleshoot and then, if required, restore WordPress website depend on why you need to restore it in the first place.
Using themes and plugins that are not kept up to date with WordPress releases is the primary reason website will break and then must be restored...
Many times, when there is conflict, you which plugin is the in the path of the message.
Stay in your dashboard and go back to your cPanel File Manager or via FTP and reinstate pluginsOFF back to plugins.
You want to check that all your plugins being tested up to the version of WordPress.
If you see similar to the one below on the plugin's page in WordPress Plugin repository — this is called clue...
You can verify if your theme is causing the problem by going to Appearance > Themes > then activating WordPress default theme.
you should be able to go to in your hosting account of the wp-content themes your-theme directory and FTP fresh copies of the files you changed that you can then FTP and overwrite the modified files on the server.
you need to bring in some help to determine the damage caused and what is required..
Then, to review that your theme and plugins are tested and compatible with each version of WordPress..
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