Here’s what I did: 

I added an extra EBS volume to an instance, mounted it on the instance, deleted the EBS volume, forgot to remove the entry from /etc/fstab and rebooted the instance. Now it tell me at boot that it cannot find the volume and I need to press a key to continue. I saw this on the read-only logs from the AWS management console. Since the instance is not up fully, SSH connections are also denied. How do I interact with the boot screen and press that key it is asking me to?! Apparently this cannot be done. 

Thanks to cyberx on ServerFault, there’s a workaround to my particular situation: I created a snapshot of the EBS volume that won’t let be boot in and then created a new EBS volume from that snapshot, attached it as a secondary disk on another instance, edited the etc/fstab file present on this disk and then attached the EBS volume back to the old instance. And that solved my issue. 

But the real issue still remains. How do I interact with the boot screen? Hopefully Amazon will give me a way to do this soon.