Problem solved with a "Power Cycle" which didn't seem to work as promised, but the keyboard is now working!!!
This morning my keyboard stopped working. The wireless mouse and Touchpad are fine. There is a yellow warning triangle in Device Manager against 'Standard PS/2 Keyboard'. I cannot remove this by updating the driver, nor by disabling/enabling the device. I followed a Dell troubleshooting guide and found that the keyboard does work outside the operating system and works inside the BIOS. It does not work when I start the laptop in Safe mode, (and obviously, it doesn't work when started in standard mode). I tried several 'fixes' from the internet:  - deleting/rebuilding 'Upperfilters' in Regedit - didn't help  - Changing the Start setting in i8042prt service from 3 to 1 or 5 - didn't help  - I also did a system restore to a previous date without success. The Yellow warning remains stubbornly in place and the keyboard won't work. Can anyone suggest a solution?
XPS 13 9310, keyboard stopped working