PEM was working for SSH , but not working now

Hello.

Have you tried the troubleshooting described in the documentation below?
https://repost.aws/knowledge-center/ec2-linux-fix-permission-denied-errors

Try troubleshooting SSH after connecting to EC2 using a serial console or Session Manager in Systems Manager.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-to-serial-console.html
https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager.html