I have setup one Application load balancer crossed zoned. Initially all instances are in healthy status but after reboot a instance that particular instance shows unhealthy in the target group.
Protocol HTTP Path / Port traffic port Healthy threshold 5 Unhealthy threshold 5 Timeout 5 Interval 30 Success codes 200
Are you sure your webservice has successfully started back up after you rebooted it?
You'll likely need to provide much more information (such as OS + webservice) if you are to get any proper answer, but as a general suggestion you your remote into your EC2 instance and figure out if the service (IIS, Apache, Nginx or otherwise) is actually running.
