Hey! So, I've noticed lots of help threads about this. Look here instead! It's usually just little things that are easy to fix.
1. Make sure your caps lock is not on. The login is caps sensitive! Make sure you're typing everything correctly.
2. Do not use the numpad! Use the numbers above the alphabet on your keyboard.
3. Does your username or password have any unusual symbols such as ?$!@*-_ ect.? This can mess things up for some people. Make a new account if you're having trouble with symbols. If you make a new one, please try to stick with a-z letters, nothing capitalized.
4. Make sure that your keyboard language is in English! It won't log you in unless you have a US keyboard setting.
5. If you're trying to log on ingame, make sure the server is up and running. You can do this by looking at the top of the banner of this forum.
6. Make sure you are logging in with your username, not forum nickname. My actual username is JazzForLife, and I log in with that. I do not use Jazzz, because that is my forum nickname.
7. Even if you change your password on the forum, your password for the game will always be the same you signed up with.
Hope this helped. If anyone has any more tips to add, tell me and I'll add them.