Just same thing happened to me too. I didn't update the app, I was going to, later, I has 2 windows open and one incognito window, total of 40 or 50 tabs, I opened an incognito tab again and suddenly windows showed the option to end process, I didn't click end but it automatically closed. When I reopened, it opened as if it was a fresh install but Ctrl+shift+t opened tabs from a day ago, since then I had opened and closed a lot of tabs but it was all gone, and it had completely cleared the other window. Even the key combination didn't bring them back, though they were open from days back. I expected the incognito tabs to be vanished but not a normal window.
This is the 2nd time it has happened, first time when I was new to opera and had no idea how to get the tabs back. This is so annoying and terrible because opera was known for being lighter and more stable than others, but this is unbearable. I need the tabs back. Or I might just shift to firefox