Permission denied (publickey) when SSH Access to Amazon EC2 instance

Permission denied (publickey) when SSH Access to Amazon EC2 instance

Hi I want to use my Amazon ec2 instancce but faced error
Permission denied (publickey).
I have create my key pair and download "pem" file.
Give
chmod 600 pem file.
Then this command
ssh -i /home/kashif/serverkey.pem
ubuntu@ec2-54-227-242-179.compute-1.amazonaws.com
But have this error.
Permission denied (publickey)
Any help please?
Also how can i connect with filezilla to upload/download files.
Many Many Thanks.