You should uninstall LR 5.x.x using the normal OS uninstall process and download and install LR 5.7.1 from the Adobe Updates site: http://www.adobe.com/downloads/updates/
What happened is you accidentally installed the CC version of LR instead of the serial-number version. The link, above, has the latest serial-number-licensed version and that should work ok.
Thanks ssprengel. I will follow your suggestion and let you know if it worked.
It worked, in a different way though. I uninstalled using the 64 bit LR setup program, reinstalled in a different location using the licensed download then run the LR executable lightroom.exe . At starting it requested the serial numbers of the initial and upgraded versions, and started up correctly again. I might have idownloaded the 30 days trial CC version as you suggest.
Thanks for your help, problem solved!