Did you recently change your domain name? Maybe an issue with the cookies?
I found the problem, but have no idea how to fix it, it's probably a mod you installed.
After you login, you get redirected to https instead of http, just by removing the s, I managed to login to your forum. It is most likely a setting either in a mysql table or in your code that you have to change.
Try disabling secure cookies.
I changed it myself and your website is working fine now. Please check from your end.
I went to phpMyAdmin, to your phpbb_config table and changed the cookie_secure value to 0 from 1. That's it. Exactly what noobie said, just that his command didn't work for some reason.