Check in the task manager to make sure that all the ArcGIS server processes are running.
You should have processes like these below after an install for server 10.1 or 10.2
-ArcGISServer.exe
-cmd.exe
-rmid.exe
-javaw.exe
If you do not have these then look into the windows services and see the ArcGIS server service is started. There is a possibility that the service is not running
To get to the services
Start > Run > Type services.msc in the run window and press enter.
This should open the windows services to see all.
If the service is running, it is possible that there could be a port conflict. check and see if you have anything running on port
6080. To check if the port is being used or not from the command line type the syntax below
netstat -ao | findstr "LISTENING" | findstr "6080"
You might also want to check with other browsers to see if the results are different
If all the above does not correct the issue, I recommend that you reach out to Esri support services for assistance.
Good luck
