I have a new computer that I just installed Windows 10 Pro on. (It's still not activated though.) It has all the latest updates.
So once in awhile when I try to log in after the box was locked (WinKey+L sequence), it gets stuck on this screen (just took the picture):

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I saw this happen already twice. The first time it happened, I let it spin like that the whole night. Then in the morning, I tried Ctrl+Alt+Del, or quickly pressing the power button. The message changed to "shutting down" but that was it.
So eventually I had to press and hold the power button for about 5 seconds to kill the power to the box. That rebooted it...
Does anyone know why does it get stuck like this? And if it does it again, is there a way to "un-stuck" it without killing the power?
PS. This is a local desktop. It has a single local Windows account that is an administrator. The Windows account has a password, but is set up for autologon during boot. I installed this Windows 10 literally two weeks ago. So it's almost mint-fresh with the stock setup.