Scan your WiFi for your Windows server and connect to it using the ServerControl app for iPhone.
Note: You can use the ServerControl app when out of the office however that requires you to access your windows machine via a VPN.
Tap the Shutdown option
Set the optional delay, and give users a message for the reason of the shutdown.
A timer will appear on your iPhone and you can cancel the shutdown at any time.
All connected users will be presented with your message.