Go to the same folder as your Lightroom catalog file. In that folder you will find a file whose name ends with .LOCK (or lowercase .lock). Delete this .lock file. Do not delete anything else.
You need to go out right now and buy a UPS (Battery Backup System, Uninterrupted Power Supply). No desktop computer should be used without one.
Thank you so much for your answer.
I worked and I am soooooo happy J