@leocg Hi!
I didn't restart it. As a matter of fact, this happened multiple times to me, and it was always the same. First and only window to open at that time was an incognito one.
I turn on the PC, I open an incognito window (maybe just after it turned on, or after half a day) as first and only Opera window and if it shows the animation that indicates Opera has been updated, I know I lost my tabs.
Always after that sequence, my normal tabs are wiped.
Which Opera? It is not GX nor Air, so I guess One? It is the basic one most people download through a Google search.