If you are running ArcGIS for Server on a VM with the Web Adaptor installed, you might find that you are no longer able to access your services after a reboot. In some environments such as Amazon, the Web Adaptor is configured against the internal IP of the machine if it is not part of a domain. As this dynamically changes when the machine is rebooted, the Web Adaptor will need reconfiguring against this internal IP as it will have changed.