Your photos/folders are NOT IN Lightroom. The Lightroom catalog is just a database that references the photos/folders on the disk. If you delete the photos/folders in Finder than the Lightroom catalog can no longer find them. If you used finder to delete the files than they are probably still in the trash and can be "put back" exactly where they came from. If not I hope you have a backup other than time machine.
Thank you for your reply. After extensive Googling, I understand that the photos were physically not in Lightroom... I did restore them back from trash - they are back in the HD. My main goal is to restore the folders/collections in Lightroom. Is that even possible? If yes, how do I do that?
All the research on the Internet has only lead me to help with restoring photos in Lightroom while the previews are still there. However, they are all gone, so nothing to start with. How do I go back from 0 to all my files/collections restored in Lightroom, please?
If you have a backup catalog from before the Delete you could try to load it and see what happens. Just make a copy of it and rename it to something else before you load the backup catalog so you don't make the mess worse. This will only work if the "put back" from trash placed the files and folders in the same place they were before. Trying this will not affect your original catalog.
Problem is, I have no idea where the files were before deletion, so I simply moved everything from trash back into HD without any specifics...
How do I even begin with restoring the Collections, when there are none left in Lightroom? (considering there are no files there at the moment...)
Do I just start from scratch again - import all files again? Then I will lose all editing...
It you have zero files or folders in lightroom you may be working in a new catalogue.
In Lightroom menus goto, 'File - Open Recent' and see if other catalogues are listed.
That would probably have been helpful, but unfortunately, I had to import a few photos and edit them after my disaster with deletion, when I opened Recent, it just reverted to these last imported photos...