Hi, my daughter had a problem with her Win 8.1 laptop with the exactly same symptoms. Can't boot into safe mode using F8 key as Win 8.1 boots way too fast...lots of stuff on the internet about this particular irritation.
Sorted it (!) by switching Laptop on using power button, then off using power button (before the black screen with mouse pointer appears) several times in a row. May have been a coincidence that after the third on/off cycle, it booted properly into Windows (with a few grumbles/messages as you'd expect). It has been fine since.
Actually got this suggestion from an IT guy at work who said he thought it might be an old wives tale and had never tried it himself.....it sounds a bit fishy, but whatever, the computer is working again!