I let this go on about 15 minutes to let it finish anything, and then I powered it off, waited one minute, and powered it back up. It took longer than usual for the Toshiba sign to go out, and for the blank screen to resolved, and then it told me it was configuring maintenance. I thought it was supposed to do this only after Windows Update. It finally came up normally.
I checked the Action Center under the Control Panel, went to "Maintenance" and it said it had finished normally at 1.14 PM, so the computer had remained frozen about 30 minutes unnecessarily. This sounds like a real Windows 8 Action Center bug.
I don't leave it on at night. It normally runs Action Center as I boot up, and it normally only takes about a second. The Action Center flag appears on the notification bar about two minutes after bootup.
I checked Windows Update and it said it had installed on Sept. 16, but I don't recall being prompted to restart with update, which you must normally do within two days. It appears that Action Center was trying to force that update to be reconfigured.
The best reference I can find on the problem now is here.
Microsoft has yet to do an adequate job of explaining what this Action Center accomplishes.