Localhost 500 error – can’t get to wp-admin

I am running wp on localhost.

I have, until an hour ago, had no issue with http://localhost/wordpress/wp-login.php

I tried to enable multi-site, backing up wp-config.php and .htaccess.

I got a 500 error when trying to log in to http://localhost/wordpress/wp-login.php.

I have restored the backed up versions of wp-config and htaccess, and still got a 500 error. I have restarted apache2, and even restarted my laptop.

I still get the 500 error.

How can I regain access to wp on localhost?