In order to access EC2 instances over SSH, a Key Pair will need to be defined.
A key pair has a public access key portion which is held by Amazon, and a private access key portion which is held by you.
Once generated, a private key cannot be regenerated, but it can be used for multiple systems.
There should be a file downloaded (or request to start a download). This file is your private key. Keep it in a secure location as it will allow access to your EC2 instances over SSH