EC2 instance attached to a load balancer is showing Unhealthy status

I created a load balancer and assigned it one of the running EC2 instance. After creation, I navigated to Target Group section in the AWS Console under Load Balancing and when I selected the target group that was assigned to the load balancer, it shows registered instance status as "Unhealthy" and there was a message above registered instance pane that says "None of these Availability Zones contains a healthy target. Requests are being routed to all targets". While creating the load balancer, I selected all the subnets (availability zones). settings I used for health check are mentioned below,

• Protocol: HTTP
• Path: /healthcheck.html
• Port: traffic port
• Healthy threshold: 3
• Unhealthy threshold: 2
• Timeout: 5
• Interval: 10
• Success codes: 200

So why does my registered instance status as "Unhealthy" and how can I rectify/resolve that to change the status to "In-service"?

Unhealthy indicates that the health check is failing for the instance.

Things to check:

• Check that the instance is running a web server
• Check that the web page at healthcheck.html responds with a valid 200 response
• Check that instance has a security group that permits access on Port 80 (HTTP)