EC2 instance remote desktop suddenly unable to access

Based on your description, there are several potential causes for your intermittent RDP connection issues to your Windows EC2 instance. Here are some troubleshooting steps you can try:

1. Verify your security group configuration:

• Ensure port 3389 (RDP) is open in your security group's inbound rules
• Check if the rule allows traffic from your IP address or the necessary source

2. Check network configuration:

• Confirm your instance is in a public subnet with an internet gateway attached
• Verify the route table has a route for 0.0.0.0/0 pointing to the Internet Gateway
• Check Network ACLs (NACLs) allow both inbound traffic on port 3389 and outbound traffic on ephemeral ports (1024-65535)

3. Verify Windows settings using AWS Systems Manager:

• Use the AWSSupport-TroubleshootRDP automation document to:
• Verify that Network Level Authentication is disabled (if not part of an Active Directory domain)
• Check that the Remote Desktop Service (TermService) is set to Automatic startup and is running
• Confirm the RDP port is set to the default 3389
• Ensure Remote Desktop connections are allowed on your instance
• Check if Windows Firewall is blocking RDP connections

4. Consider resource constraints:

• If you're using a smaller instance type (like a t2.micro), it might be experiencing performance issues due to CPU credit exhaustion or resource limitations, especially after periods of inactivity

5. Authentication issues:

• Ensure you're using the correct username (typically "Administrator") and password
• If you changed the administrator password after initial setup, the password retrieved from the .pem file will no longer work

If the issue persists, you might need to stop and start the instance (not just reboot) or consider using Session Manager as an alternative way to access your instance if it has the SSM agent installed.
Sources
Troubleshoot issues connecting to your Amazon EC2 Windows instance - Amazon Elastic Compute Cloud
Unable to connect to my Windows Server instance through Windows Remote Desktop | AWS re:Post
RDP can't connect to running ec2 G instance | AWS re:Post
Free EC2 doesn't work | AWS re:Post

Besides RDP from internet, you can try connecting in-browser using Fleet Manager Remote Desktop.

Here is a video that walks through the configuration and connection process

Confirm the RDP port is not blocked, check if there was no recent security update or activity that required port level block