Amazon Linux AMI, the user name is ec2-user.

RHEL5 AMI, the user name is either root or ec2-user.

Ubuntu AMI, the user name is ubuntu.

Fedora AMI, the user name is either fedora or ec2-user.

SUSE Linux AMI, the user name is either root or ec2-user.

 
Otherwise, if ec2-user and root don’t work, check with the AMI provider.