Partly followed some of advice I read, just pressed FN button and then left arrow screen at same time, the screen brightened instantly. Didn't save or do anything else, so was very simple. Problem occurred after updating to Windows 10. So relieved this has worked, as looked like a serious problem... Was very inconvenient. Thanks for advice, hope this useful to others. Update (12/04/2015): That should have read press FN button and left arrow KEY at the same time.