I uninstalled on old USB monitor program. All was OK until the uninstaller asked whether I should reconfigure USB ports using (something I can't recall now). Anyhow, both the mouse and the keyboard were not responding obviously because this uninsteller somehow switched off the USB ports. So I had to force a shutdown by pushing the power button.
Now - naturally - I can't log in to my original OS without mouse/keyboard. Obviously there are no physical problems with USB ports: I can log in to my PC using a linux or winpese-x64 usb stick. With those two there are no problems with USB ports.