I\'ve created an instance of a server on EC2 based on an AMI generated from an existing server. All goes well during the create, and I specify the same key for the new server as
One Reason i found is entry in WLAN in current working network i.e. of my office. you may have restrictions.try contacting network admin.
alternatively you can try for adding entry to inbound rule of your current ip address.
It did turn out to be a security group/permissions issue. The default security group looks open, but actually shuts everything down, per this post:
https://serverfault.com/questions/245916/why-cant-i-ssh-or-ping-my-brand-new-amazon-ec2-instance
as you are taking the existing AMI you have to delete all the old entries from authenticated file except new entry. because while the coping the AMI old entries are still present in new instance so once you will delete it you may able to login into the instance.