Tried umpteen of these solutions for hours.
Eventually logged into amazon and trouble shooted Kindle e-readers - it found my device, (Kindle keyboard - if you are unsure, look on the back of your device at the bottom and find the model number - google it and it will tell you - however, when I logged into my amazon account it knew which model I had).
So held the power button for 40 seconds (who knew 10 seconds could make such a difference) - initially nothing, so went to next stage and plugged in to power - low and behold it rebooted. Took less than 5 minutes to solve the problem I had spent hours trying to sort.
Moral of the story - why google when you can trouble shoot at the manufacturers website.