While most people have no trouble to login, it's hard to find out what is wrong. If you have problem, I suspect your computer is not quite compatible with the dungeon cookie settings. You can do the following check:

If possible, you may want to try it on another computer, even if they are on the same network. So we can make sure it's only specific to your current computer.

Make sure you are using correct username and password. Try to logout the forums and log back in. Dungeon uses the same username and password as in the forums.

Make sure your browser cookie settings are not touched, or make sure cookie is enabled in your browser.

Make sure do not use AOL browser. IE (version >= 5.0) is the best choice.

If you can access before, check what software you installed recently. Antivirus, browser toolbar, firewall, security software could all mess up your cookie setting.
For security reason, all the form submissions (that is, including the login form) in dungeon are checking the HTTP_REFERER variable sent by your browser. Some firewall software like ZoneAlarm strips this variable from your web traffic. You need to disable it before you can go pass the form.

I hope this can help. I will put this into the FAQ section of this site.

Jinn