There's a certain way about handling keypairs. Let's say in this example that we create a keypair called "1stKey". When I click "Create Key Pair" it asks me for a name and generates that key, then causes my browser to download the private key named "1stKey.pem". EC2 will never let me get that private half again, so unless you can find your
$whatever.pem file, you'll need to generate a new one. You can also upload the public half of an already existing key.
From there, you'll do the usual
ssh firstname.lastname@example.org -i path/to/your_key.pem. If you don't have the key that was used to create the instance, you'll need to
terminate it and start fresh mount that EBS image to some other functioning instance and fix it or kick yourself for not using an EBS backing. At least for the default Amazon instances, there is no direct root access, but rather sudo access for ec2-user. Perhaps the people who generated your image followed that convention.