(An administrator successfully accesses an EC2 instance via SSH from a local computer then stops...

Comprehensive and Detailed Explanation From Exact Extract:

After stopping and starting an EC2 instance, the public IP address may change (unless an Elastic IP is attached), but the security group rules remain intact. If the instance is no longer accessible via SSH, it’s likely due to an inbound rule (e.g., for port 22) not being correctly configured or applied. Adding or verifying a rule in the security group associated with the network interface (NIC) to allow SSH (port 22) from the administrator’s IP resolves this. The WGU Cloud Deployment and Operations Study Guide (Section 3.2, Security Groups) states, "Stopping and starting an EC2 instance may require verifying or adding an inbound SSH rule (port 22) in the security group if connectivity is lost due to IP or rule misconfiguration." Route table or IP changes are not the primary issue here.