The only thing that I could recommend is to export your bookmarks regularly in the future, so you can restore them from there or transfer them to another device.
Sorry, I don't have any better solution by now, but I'd like to support your request because I have had similar problems on various occasions:
when synchronizing, it went the wrong way: it did not update the stored bookmarks with the new ones but removed the new bookmarks on the computer, so the former version was restored on it (although adjusted the "right way" round in Settings).
It happened too many times so I don´t consider synchronizing a safe process on Opera. It might function sometimes but I think it´s better to have your bookmarks exported and stored.

There is an extension called Tab Hamster which I strongly recommend. It restores all lost tabs with one click.
Losing all tapes has become sth regular during the last year, but this time it happened two times within 24 h.
Not the only fault. I don`t know what is happening but Opera is getting worse and worse, losing all its former advantages. Been using Opera for decades, so it is hard to leave, but I am looking now for alternatives.

@xb70 try Vivaldi

@xb70 try Vivaldi

@ledsalesoz Came back to thank you again for bringing up Vivaldi. I´m really happy to have such a rich, working! browser at hand.
A browser is such an important thing for everybody on the net, so it was a hard decision to leave the one I used such a long time. Now that I´ve been working with Vivaldi for months, I think it is even better than Opera ever was. And as for the workspaces: it has them. Only to arrange them the way they are presented on Opera you have to work a little bit. Hope that answers your question. If not, dm. All the best!

@markoze When still on Opera I used Tab Hamster which worked fine. But with all the faults on Opera I´d recommend to consider changing to Vivaldi (only came back to this thread to thank the person which recommended it).

@ledsalesoz Thanks a lot! for bringing up Vivaldi. Tested it the last week. It works perfectly and definitely will substitute Opera on my new notebook. Lost my tabs about four times more, and although TabHamster makes it easy to restore them, I won´t waste any more time on a browser with so many bugs and questionable data security.

@sentaberger Go to Browser History in the Opera Menu, In case it is still intact you can restore your tabs from there. Good luck!
There is an Extension named TabHamster; when installed you can recover lost tabs with one click only. I strongly recommend you installing it or another similar extension because losing all open tabs might happen again.

The only thing that I could recommend is to export your bookmarks regularly in the future, so you can restore them from there or transfer them to another device.
Sorry, I don't have any better solution by now, but I'd like to support your request because I have had similar problems on various occasions:
when synchronizing, it went the wrong way: it did not update the stored bookmarks with the new ones but removed the new bookmarks on the computer, so the former version was restored on it (although adjusted the "right way" round in Settings).
It happened too many times so I don´t consider synchronizing a safe process on Opera. It might function sometimes but I think it´s better to have your bookmarks exported and stored.

There is an extension called Tab Hamster which I strongly recommend. It restores all lost tabs with one click.
Losing all tapes has become sth regular during the last year, but this time it happened two times within 24 h.
Not the only fault. I don`t know what is happening but Opera is getting worse and worse, losing all its former advantages. Been using Opera for decades, so it is hard to leave, but I am looking now for alternatives.