How to set up SSH access to EC2 running Ubuntu

I launched an EC2 instance running Ubuntu and during the launch I generated a key pair and downloaded the private key to my home computer. I was able to SSH from the command line to my EC2 instance and also access it using FileZilla. Now I have replaced the OS on my home computer although I did save the contents of the private key before doing so.

I created a file in: ~/.ssh/filename.pem which I pasted the saved private key into (filename was the same as the original filename). When I run ssh -i ~/.ssh/filename.pem <server IPv4 address> I get the following output:

$ ssh -i ~/.ssh/ec2-1-key-pair.pem 13.41.164.129
Load key "/home/steve/.ssh/ec2-1-key-pair.pem": error in libcrypto
steve@13.41.164.129: Permission denied (publickey).

When I try to associate the .pem file with a site in FileZilla, I get a warning saying something like "Can not use file because it either does not contain a key or it does not exist (I do not have access to the exact wording at the moment).

Please can you tell me how to get SSH running on my new OS. I do have access to my server using AWS Instance Connect.