It seems like you might have done some syntax error etc please add these lines to wp-config.php to check error
If you have backup of your previous file, restore it to check error if any are there.
error_reporting(E_ALL);
ini_set('display_errors', 1);
define( 'WP_DEBUG', true);
500 errors can frequently occur with incorrectly configured rewrite rules contained in your “.htaccess” files, so you should probably try removing those from your folders to see if it’s the cause. Keep a backup of those files of course, you might still need them. But in your case it was working before and you had made changes in template file so reason seems like syntax error.
Thanks!