The best thing to do would be to press the power button on the computer, it would either shutdown the computer or hibernate depending on the power settings.
You can then restart and see if it starts normally or safe mode can be selected at startup. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
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Once you are in Safe Mode follow the method 6 to uninstall the update causing the trouble. Insert the Windows 10 bootable installation DVD and restart your PC. When prompted to Press any key to boot from CD or DVD, press any key to continue. Last time mine did that (a week ago) I did a hard restart, when it restarted, Windows System Restore automatically fired off during boot and rolled the system back, then I did a windows update again, this time I installed the updates one at a time. Would you like to answer one of these unanswered questions instead?Fix Working on updates 100% complete Don’t turn off your computer: Windows updates are a crucial part of System which ensures smooth system performance. Configuring Windows updates 100% complete Do not turn off your computer. If you don't have an LED, put your ear to the machine and listen for disk activity.Similarly, if you see heavy blinking on your network card, it may just be downloading some delicious MS-certified spyware (etc).