There are any number of reasons why this could be happening, but your scenario is more than likely to be one of the following three:
If you haven't been active on the site for a really long time (like, months), and you never posted, your account may have been deleted. (If you did post, your account is probably still there, since we usually only delete 0-posters.)
Another possiblity could be attributed to a glitch that happens occasionally... If you try to log in, and it doesn't let you, but the 'who is online' thing says you *are* on, don't worry, no one is lurking around on your account. Try logging in again, and it should work. If not, delete your cookies and try again. (This is usually the problem, and deleting your cookies almost always fixes it.)
If you just registered and you can't log in, it's because you didn't click the link in the registration e-mail the forum sent you. If you didn't get an e-mail from the forum, it's probably because you didn't provide a valid e-mail address. Re-register with a new username and a correct e-mail address, and then PM me so I can delete the old username.
There are, of course, other reasons why you may not be able to log in, but those are the most common (and most likely). If you don't think your problem matches any of those listed, e-mail me for help and we'll figure it out. (But please, make sure you absolutely can't do anything to log in! I don't have time to help you if your problem is one of the above mentioned, and you just didn't figure that out.)
Don't bother sending me messages or anything, I'm not in charge of the site anymore.