This error occurs because your WordPress site could not load the file from the code. Your blog may continue to load with the error message or may fails to load with error message saying fatal error.
You will see the error messages that tells you where the error is located and can be fix following the clue in the message e.g. such file or directory, permission denied, fatal error.
Warning: require(/home/blog/wp-includes/load.php): failed to open stream: No such file or directory in /home/blog/wp-settings.php on line 17.
This guide could help you to fix the error. https://www.webminto.com/how-to-fix-the-wordpress-failed-to-open-stream-error/