Everyone knows that when all else fails the answer is a power-off reset. Trouble is that with the Kindle, the battery is permanently connected and not easy to remove so this facility is not available to Kindle owners. However, like all computer based equipment the running software is liable to occasional corruption. This happened to me like many others and the highly efficient Kindle help desk advised me as above: Hold the power switch on for over 10 seconds and an automatic complete reboot will be initiated. It was and my Kindle is now working again as it should. Clever bit oif kit as well. A book I wanted to purchase was downloadad in about 5 seconds.
Of course this is described in detail in the Kindle instructions .... which you can't read because your Kindle won't work!
P.P.