EC2 Suddenly stopped responding

Hello.

Is the EC2 IP address used during SSH connection correct?
If you have not set an Elastic IP address, the public IPv4 address will change when you restart EC2.
https://repost.aws/knowledge-center/ec2-recover-ip-address

Also, try connecting to EC2 using Session Manager instead of SSH and check if sshd is running.
If SSM Agent is installed on EC2 and the appropriate IAM role is set on EC2, you can connect with Session Manager.
https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager.html