Microsoft says:
We are investigating reports that a small number of devices may startup to a black screen during the first logon after installing updates.To mitigate this issue, press Ctrl+Alt+Delete, then select the Power button in the lower right corner of the screen and select Restart. Your device should now restart normally.
We are working on a resolution and will provide an update in an upcoming release.
I've had it happen on 3 devices so far -- as luck would have it none of the 3 had a keyboard attached, so I can tell you if you wait it out you'll *probably* eventually get a display.
I always run Disk Cleanup afterwards to get rid of the Windows Update garbage, & it usually amounts to 1-3 GB worth. 3 times so far it hasn't worked, not finding & listing the Windows Update junk. Win10's footprint on disk is of course going to be larger than previously when that happens.