EC2 Linux Instance is up but not reachable

Take a look at the Cloudwatch metrics for the instance to see if CPU usage is high, thereby making the machine unresponsive - https://aws.amazon.com/premiumsupport/knowledge-center/ec2-linux-resource-over-utilization/. You will need to monitor the processes to see which process may be driving up resources usage to the point where the machine eventually becomes unresponsive.

You can also install the Cloudwatch agent on the machine to send logs to Cloudwatch logs - https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/Install-CloudWatch-Agent.html