What version of PHP is your website running on? If you’re running on a low version of PHP, switching the PHP version might fix the issue.
Otherwise:
Fix 1: Turn on WP_DEBUG mode from your wp-config.php file, then visit your website to see which file/theme/plugin is causing the issue.
Fix 2: Rename your plugins/themes folder name one after the other and refresh your site, if the error is coming from any.
Fix 3: Manually downgrading to a lower version of WordPress – note that this is not the best approach but if you need to get your website back up and running ASAP!
