To access (via SSH) an AWS EC2 Instance, you need the key that was generated when you created the EC2 Instance. This key should have been placed in a safe place, backed up (several times) etc. However, if you have lost your key, suffered hard disk failures or otherwise just been a bit clumsy, then you will no longer be able to access your EC2 Instance.
You cannot recover lost AWS EC2 Key Pairs, so stop looking for that menu, it's not there. However, we can follow an almost identical process to this Move EC2 Instance. In the Move EC2 Instance guide, we are moving an existing EC2 Instance from one Availability Zone (AZ) to another. In the process of the move we have to generate a new Key Pair, allowing access.
So this means that if you make a copy of your EC2, which you can do from the EC2 Dashboard, you can then create a new EC2 instance from that copy, that is exacly the same as the old EC2 Instance, in this process you can create a new Key Pair that I am sure you wont lose this time :).
The process is quite simple, and it goes like this:
Create an AMI (Amazon Machine Image).
Create new EC2 Instance from the AMI.
Create new Key Pair.
So use the Move EC2 Instance guide and just ignore the part about changing Availability Zones.