If you are paranoid about security, you may have set up a security rule restrict the RDP traffic source traffic.
If you get a different public IP from your workstation (due to location change or release of IP address), you may no longer be able to access your AWS EC2 instance remotely.
To troubleshoot, you can login to AWS console, view your running instance. Under Security groups, choose view rules. You can now see the allowed source IP address for RDP traffic. Once you update the security group rules with your new public IP, you should be able to connect without reboot the instance. Note that the security groups rules may not reflect your new public IP right away.