I was able to resolve this.
let me rephrase the problem first, as directed, i would create and launch a EC2 instance and eventually the jupyter Notebook and do all the work and the end of session stop the instance.
next day while starting again, i would try to start the instance (which would start with aws-start and the steps before) but it would connect to the instance with above error!.
ok, here is what i checked:
the keypair name in my Amazon console is aws-key
and then i run the command (instance is already running, as can be seen via web console) :
$ ssh -i “~/.ssh/aws-key.pem” ubuntu@$instanceIp
and viola… its running again.
after several days of frustration now i can work, save and then start again from there.