If you're having trouble logging in all of a sudden, but you've been able to log in okay previously, it's possible you've had five failed login attempts in one way or another. The main symptom of this is if you can't login using yourdomain.com/cpanel, AND you can't view your site, or webmail.
If you try to login five times unsuccessfully, the server will try to protect your site by completely locking out your IP address for an hour. Sometimes it's not your fault, as email and FTP programs can make this happen occasionally.
If you don't recall trying to login five times, try disabling your email and FTP programs temporarily, and try logging in again in an hour. If that doesn't work, please open another chat, or send an email if you can, to firstname.lastname@example.org, and we'll dig a little deeper with you. You can try on another computer with a different IP address in the meantime.
Occasionally, although it's pretty rare, this may also simply be temporary problem with your ISP, almost like a "blind spot". It's possible you'll even be able to see other sites on the same server, yet not your own. The good news is, it will clear up on its own. Of course, let us know if it doesn't, but it's difficult to say how long it will take. It could be minutes or it could be as long as overnight, although often it clears up within a couple of hours. Again, you can try from another computer, and it will most likely work fine.