I killed ssh daemon by mistake. Any chance I can connect to the instance and start it again?
If a reboot does not work and you are running an EBS boot AMI/instance (recommended), then you can get access to the root disk of the instance by attaching and mounting the EBS volume to another instance, looking at log files for errors, fixing the system, and then moving the volume back to the original instance.
I provide specific instructions on how to do this here:
Fixing Files on the Root EBS Volume of an EC2 Instance
If you are running an instance-store AMI/instance (not recommended), then you are in trouble and may need to beg Amazon through your premium support contract to see if there is any chance they can make the root disk information available to you.