Reason: This occurs due to the disassociation of your elastic IP.
Attach a new elastic IP to your AWS instance.
There are two ways to resolve this issue,
1. Create a new elastic IP and then attach it to your instance.
2. You can request AWS to assign a new public IPV4 address to your instance.
For that, Go to instance>>Actions>>Networking>>Manage IP address
Click on Assign new IP will assign you with a new public IP from AWS.
For more information check out this aws tutorial.