Why is PuTTy SSH client no longer connecting to EC2 instance?

I had been using my instances private key file (but converted to .ppk for开发者_运维知识库mat for Putty) to ssh into my ec2 instance. However, after generating the cert key file from the management console, the putty connection stopped working. It would keep telling me the network connection timed out, or that I just get access denied. I don't know why this would suddenly change, as I didn't change anything with the instance itself or the original private key file.

Your comment said the answer.

Once an EC2 instance is launched, it's assigned a public and private IP address at boot time. An instance's private IP address will never change during the lifetime of that instance. However, its public IP address can change under certain circumstances.

Note: Public and private DNS names are constructed based upon an instance's public and private IP addresses. So, if an instance's public IP address changes, the public DNS name will also change accordingly.