Verify your account Login details twice, if you have typed it correct. Dont copy paste user names and password.
Webmails is accessed via http://yourdomain.com/webmail
User name for Webmails are usually your full email id and not just the user name of the email. Kindly enter your full email id.
Cpanel is accessed via http://yourdomain.com/cpanel
Login details that you would have got in your welcome email