How to enter the server by key after reinstalling the OS?

Question

Amazon Web Services

I saved the key issued after the creation of the server to the flash drive. Before reinstalling the OS, I went to the server without any problems. And after the attempt, Permission denied (publickey) ends. I read a lot of information but nothing helps.
I go like this ssh -i "key.pem" [email protected] Changed the root user on the advice of ec2-user, changed the rights to the key, whatever he did. Still can't login to server running on Centos 6.7 I have Xubuntu installed