A couple things I would check would be are you copying the correct Public DNS name to filezilla?
Host: Public DNS
Login Type: Normal
User: ec2-user (as you said)
However you also have to apply the keyfile. Go to Edit-->Settings And under connection go to SFTP and add the keyfile associated with your instance. If you have protocol set to SFTP you dont have to put port 22 as it will automatically connect through port 22.
Also if you assigned the instance an elastic IP address AFTER you logged in the filezilla, the Public DNS will change.
If none of this helps, what is the exact error message you are getting?