Just in case anyone is interested (and I did say I would post again once I found out exactly what she'd done to get her laptop working again):
1) Powered up laptop using power button, waited until it booted as far as it will go (i.e. black screen with mouse pointer showing).
2) Switched off laptop by holding power button down until laptop shut down.
3) Repeated steps (1) and (2) three more times. On the third repeat - the fourth boot-up - Windows started and sorted itself out.
I have since passed on this potential solution to someone else who had a Win 8.1 laptop with identical symptoms. It solved his problem too.
