You Entered the Wrong Username or Password
This is the most likely explanation. If you are getting a message like "invalid username/password" simply click on the link that says "I forgot my password" and then follow the steps so that the board will email you a replacement password. If you no longer have access to the email address you used when you signed up, the easiest thing to do is to simply create a new account. If you want to use your existing account, then send us an email to the following address explaining your problem:
You Have Updated Your Email Settings on Your Profile
I believe the board automatically deactivates your account when you change/update your email in your profile. At least it did so years ago -- there have been many updates to the board software since then, so I'm not sure how it works now. In any case, if this has happened, and you are unable to log-in, shoot us an email at the above address.
The Board is Down
Websites go down every now and then. This is normal. But if this is the case, you wouldn't be able to read this page right now.
You Have Been Banned
If you are getting an "access denied" error, then this might be the culprit. Though we rarely ban anyone. If you have been a real jerk to people lately, or if you have been posting spam, then you may have been banned. Send us an email to the address above and we can discuss it.