Dear Old Toad,
I appreciate your time and efforts to answer my question,
now let me answer yours:
1- What system and Photos versions are you running?
MacOS Mojave , all the applications were updated according to this OS version.
2-Did this begin after you performed an update or upgrade? If so what?
it is not clear when exactly that happened , but it looks like it occurred after an update.
3- Normally by default the Photos library will be located in your Home/User/Pictures folder.
The folder was empty.
4- Launch Photos with the Option key held down and see if any libraries are listed in the Choose Library window.
There was no a single library existed there.
4- Do you have Time Machine enabled? Do you have a backup of your hard drive made prior to this happening? If so restore the library in the Pictures folder from the latest backup that has a library in the Pictures folder (assuming that is where you normally had it located).
Fortunately the answer is yes. I have followed your suggestion and it does work but sadly , I have managed to retrieve only the backed-up photos while the unbacked up ones have disappeared.
Thank you again Old Toad for your helpful answer