Originally Posted by
DYKWIA
You didn't save your work, and Windows Update restarted your PC? No sympathy I'm afraid. It gives you lots of notice
No, it doesn't... not on Windows 10. If you step away from your computer for a couple hours in the afternoon, you might find it reboots in the early evening without you seeing any notification.
Installing updates upon reboot is fine... initiating an automatic reboot to install updates is completely unacceptable. Fortunately there's a way to force Windows never to reboot itself automatically to initiate an update, which is the way Microsoft should have set it to behave by default.
gpedit.msc
Local Computer Policy --> Computer Configuration --> Administrative Templates --> Windows Components --> Windows Update, then set "No auto-restart with logged on users for scheduled automatic updates installations" to Enabled.