Step 1. check your portal machine is running and portal windows service is running and you can open portal using full machine name from portal machine instead of using DNS domain name
Step 2. open portal using full machine name from portal machine instead of using DNS domain name
2.1 change windows default browser from IE edge to Google Chrome
2.2 open portal with full machine name instead of DNS, if it runs fine, means your portal is ok, the problem is DNS, which is somthing wrong with your portal web adaptor machine.
Step 3. connect DNS portal web adaptor machine, found that machine is down. Reason found !
