There is lot of possibilities for this type of error . You can try below steps one by one.
As a first step check what is really going on while establishing ssh connection. Ex:
1.ssh -v -i /home/cent/keys/my-key.ppk firstname.lastname@example.org
2. Check the public and private keys are proper ,correct pair , also check the permission of the keys.
3.IP of the AWS server may be blocked by firewall Ex, on-premise firewall,Godaddy server etc.
4.check the login name for the user ,because each flavor has different default user login name in AWS.
5.Stop and start the EC2 instance or reboot
6. Check the server status in AWS console , is showing 2/2 green .
7.If nothing worked out above step in worst case scenario , stop the EC2 instance and detach the root disk , and attach any other linux instance , you can update keys properly again or user password .