What would happen if you turned off your PC (for example a power failure) during installing Windows updates when it states Do Not Turn Off Your Computer?
I'll tell you one thing, you don't want that to happen to your domain controller. Happened to me a month ago. I was using Acronis for backup software... That didn't want to restore. As the only admin, that meant a few days of solid work for me.
It depends exactly what update it was and what it was doing at the time. In most cases, either the update doesn't fully install (corrupted) or it simply resumes on the next startup or shutdown. If the installation is corrupted, Windows is usually able to detect it and either revert the changes, or request that you manually re-install the update.
And depending on what file(s) were being updated/modified at the time, you could end up with a system that will no longer boot forcing you to go to the DVD to revert to a previous restore point or perform a repair/reinstallation.
The update is not installed successfully
you can use a restore point, Select the date prior to when the computer was turned off during the updating process. Normally windows update has to create a restore point prior installation.