If it’s not the theme, it might be a plugin issue. In a way, troubleshooting is similar. However, it’s much easier to rename the plugin folder to ‘pluginsOFF’. Visit the site, and log-in. This will turn off all of the plugins. Please note that it won’t remove the original settings of those plugins, as they will be there when you reactivate them later on.
Once the plugins are off, go back and rename the folder back to ‘plugins’. Go to your WordPress admin area and reactivate each, one-by-one, until you get the screen that says “There has been a critical error on your website”. The plugin that you just reactivated, is the problem.