If you see errors like these while accessing your website:
Or after enabling PHP errors display, the possible reasons are:
Website migration: in such a case, try changing your PHP version, it should match with your previous hosting PHP version. If that doesn’t help, try to re-upload your website files.
Updating your WordPress version: if the errors appeared after WordPress upgrade, then either one of your plugins or themes uses outdated functions, that are not supported by WordPress anymore, or something went wrong during the update. The best solution would be restoring your website from backup and trying to update the WordPress version again. If the issue occurs again, then you will need to find a different plugin or theme instead of an outdated one - or use an older version of WordPress (not recommended for security reasons).
Updating your theme/plugin: same as above, you will need to restore your website from backup and do changes again. If the plugin’s or theme’s update brings errors again, you will need to either reinstall or find an alternative for them.
The other steps that can help you: