Arnab Check the log file, Flarum errors are logged to (<flarum>/storage/logs/flarum-<date>.log) and take a look at the full error message there or enable debug in (<flarum>/config.php) and try again and/or the browser javascript console for errors.
Meanwhile, I guess this is assets problem, so, as a quick fix also try to clear cache inorder to rebuild assets. Clear the cache through the admin dashboard or by running php flarum cache:clear
