What version of LR.
What OS are you using.
Have you opened the File Manager program for the OS you are using and checked if the drive the images are on is visible, that is shows up in that File Manager?
Thanks for your quick replay
I have lightroom Cc . On a cloud.
I have windows 10.
I understand what happened:
I copied part of the files to another external memory.
And it's confuesed the software: it didn't recognized the one that I am
working with but he recognised the new backup -the new external memory.
So what I did is to copy all the filess to the new external memory.