I can't connect to my EC2 instance

There are several potential reasons why you might be unable to connect to your EC2 instance after successfully connecting previously. Let's go through some troubleshooting steps:

1. Security Group Configuration: Ensure that your security group still allows inbound SSH traffic (port 22) from your current IP address. Your IP may have changed since yesterday. Also, verify that the security group is correctly associated with your instance.

2. Instance State: Check that your instance is still in the 'running' state. Sometimes instances can stop unexpectedly.

3. Network Configuration: Confirm that your instance is in a public subnet with internet access and that there haven't been any changes to the VPC, subnet, or route table configurations.

4. EC2 Instance Connect IP Ranges: If you're using EC2 Instance Connect, make sure your security group allows access from the EC2 Instance Connect IP ranges for your region.

5. Instance Health: Check the system logs and instance status checks in the EC2 console for any issues.

6. SSH Service: The installations you performed (PHP, NPM, Git, Jenkins, Apache2) shouldn't directly affect SSH access, but it's possible that a configuration change during installation might have impacted the SSH service.

7. Firewall Settings: Check if your local firewall or any security software is blocking outgoing SSH connections.

8. Try Alternative Methods: If possible, attempt to connect using AWS Systems Manager Session Manager as an alternative method to diagnose the issue.

If none of these steps resolve the issue, you may need to:

1. Reboot the instance from the AWS console.
2. If rebooting doesn't work, try stopping and starting the instance (note: this will change its public IP if you're not using an Elastic IP).
3. As a last resort, you might need to launch a new instance and attach the existing EBS volume to recover your data.

If the problem persists after trying these steps, it would be advisable to contact AWS Support for further assistance, as there might be an underlying issue with the instance or the AWS infrastructure.
Sources
Can't connect to ec2 instance | AWS re:Post
Suddenly I can't connect to my ec2 server | AWS re:Post
Troubleshoot issues connecting to your Amazon EC2 Linux instance - Amazon Elastic Compute Cloud
Can't SSH into EC2 instance anymore | AWS re:Post

There are few steps you can take to debug the EC2.

• Check security groups of the EC2 - does it allow SSH.
• Are you using SSM Session Manager - if so, check if IAM role is attached and EC2 has outbound 443 in security group.
• Check instance screenshot of the EC2 and see if it's look alright.
• Do you have user id/password, you can also login with serial console if enabled.
• Are you using private or public IP to connect. If public IP, make sure the EC2 has internet connectivity.