Unplug all USB things and power up to see if it completes. I've seen this fix too many.
Windows 10 initiated an upgrade today that only completed to 95%. Sat there for hours. Finally I rebooted the computer since it was not responsive. Since then (20 reboots later) sometimes I get to a login screen and can log in, but computer just sits and spins at the login Welcome. Any ideas?

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