- You have visited the wrong URL. Ensure that the URL is http://YOUR_DOMAIN/webmail (webmail) or http://YOUR_DOMAIN/cpanel (cpanel)
- If you enter your password incorrect after 3 attempts, you will be temporarily blocked from accessing the server.
- You need to clear up your browser cache. (see below)
- Your computer is infected by virus/malware (see below)
- Your IP address has been blocked due to security breach (see below)
How to clear up the Browser Cache:
- Firefox: https://support.mozilla.org/en-US/kb/how-clear-firefox-cache
- Google Chrome: https://support.google.com/chrome/answer/95582?hl=en
- Internet Explorer: http://windows.microsoft.com/en-in/windows7/how-to-delete-your-browsing-history-in-internet-explorer-9
For the case where your computer is infected by virus/maware you need to scan the computer you use to access the Internet with at least two anti-virus programs. If you would like help with cleaning malware from a personal computer, this forum post to gives an in-depth description of things to consider:
http://forums.majorgeeks.com/showthread.php?t=35407
If you're sure the username/passwd you're using are correct and have tried clearing browser cache but the issue is not resolving then you need to provide your IP Address to your web hosting company for review. You can get this from http://whatismyipaddress.com/