Firstly you need to search for your old Lightroom Catalog. Using your OS carry out a search for a file with the suffix "lrcat". Hopefully this will yield your catalog. Open this and all your work will be visible. If you find more than one catalog I'd start with the most recent one.
If you don't find any then you need go back to your backup catalogs which you hopefully kept on a separate disk in case of just such eventualities. Copy the most recent one back to your computer and open this with Lightroom.
Thanks a lot for your answer. I really appreciate your help.
Unfortunately, I already looked for any Ircat doc but nothing appears on my computer...
I checked on my separate disk but apparently, I did not save all the pictures with their metadata. There're just raw. And yet, to make sure I did not take any risk, I checked if the backup was efficient a few months ago: when I connected my disk to my computer, I could read the pictures on Lightroom with their filters, presets, etc. Today, I realize that the Ircat were not saved. The raw pictures were automatically changed by the software itself, I guess. In a nutshell, I don't have any backup (and it seems I never really understood how to "double save" my work...).
How could these Ircat disappear like this?
I guess that if I don't find them, there's no other way to recover my work, right?
By the way, I'm sorry if my English is approximate, I hope you understand my explanations...
Thanks again for your help
Your English is fine, certainly a lot better than my anything else!
If the lrcat file(s) really is/are gone then yes I'm afraid your edits probably are lost. As you have surmised, the way LR works is to store a database of all the changes you have made to your raw files. If you don't also save these changes to xmp side-cars and you lose the catalog then Lightroom cannot tell what edits you've done to your files.
Check your recycle/trash bin for lrcat files and also, before you do any more work on the computer use some recovery software to look for the possibly deleted lrcat file. Just Google for recovery and you should find many such software out there, some free. They will search for files that have been deleted and it will help if you know part of the file name though you could just look for lrcat, or since deletion often means file name corruption just part of the file name.
How did the lrcat file get lost/deleted? I don't know, by accident? Virus? Each is possible. This really does stress the importance of regularly backing up not just your raw files but also your Lightroom catalog.
I wish you luck.
Even though I may not find any possible way to solve this problem, your help has been precious. Thanks to you, I did not feel alone in this issue.
I have downloaded Recuva, which is currently looking for any *Ircat doc but seems compromising...
I'll find the right way to back up my work from now on.
Thanks again for everything.