My model is an Inspiron 15 7537 with touch screen, but I suspect this impacts other models.
The problem resolved itself after 2 manual restarts, the second of which included a number of automatic updates.
After the Windows 10 update, Windows seemed to think my PC was a tablet. The built-in keyboard would not respond (the touch pad still worked). Disconcerting when you are trying to log in for the first time after the upgrade. A USB keyboard worked, but without it, I had to enable the touch screen keyboard. Ironic that this would happen with a new OS expected to restore a more traditional PC interface (we haven't all moved to tablets). I found nothing on the Microsoft or Dell support pages about this, and Dell phone support was no help. There was one reference on a support page for pre-release Win 10 that alluded to just waiting for the updates.