EC2 instance attached to a load balancer later 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: HTTPPath: /health.htmlPort: traffic portHealthy threshold: 2Unhealthy threshold: 2Timeout: 2Interval: 5Success 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"?