Hey!
You can reverse the changes made on the url from the database of your site. You can find the database name listed in your wp-cofing.php file. Then if you are using CPanel, the database would be stored in the PHPMyAdmin. Find the database and in it the wp_options table. You need to locate the site url and home fields to edit the url of your site (make sure both match perfectly). That should bring back the old look of your url without www. Ideally, this will bring back the /wp-admin (/wp-login.php) page so you can login in the Dashbaord.
It seems unusual that your wp-login.php page disappeared when changing the url with www, so you may have some plugin interfering this change or something else was changed which resulted to this error.
If you after all decide to switch to www again, make sure to refresh your Permalinks to avoid the site not recognizing the new version of the url. That is easily done from the Dasboard->Settings->Permalinks and simply click the Save button without changing anything.
As a last resort, you can restore your site from a backup to a previous state when it was working properly. If your hosting does not include backups, you need to make sure to install a Backup plugin ( there are a lot of free ones out there) and backup your site before making such changes to avoid issues in future.
I hope this information was helpful.
Regards.
