There are several reasons why this could occur. First, ensure your username and password are correct. If they are, contact the board owner to make sure you haven't been banned. It is also possible the website owner has a configuration error on their end, and they would need to fix it.

You may not have to, it is up to the administrator of the board as to whether you need to register in order to post messages. However; registration will give you access to additional features not available to guest users such as definable avatar images, private messaging, emailing of fellow users, usergroup subscription, etc. It only takes a few moments to register so it is recommended you do so.

Don't panic! While your password cannot be retrieved, you can send a short message using the support page, to the team and they will reset everything for you, you will be back on in no time at all.

It is possible the website owner has banned your IP address or disallowed the username you are attempting to register. The website owner could have also disabled registration to prevent new visitors from signing up. Contact a board administrator for assistance.