This has happened several times, usually when I'm opening it for the first time in the morning although it has happened once after a crash. The "restore previous session" option under the History menu is grayed out and so isn't an available option. There's no indication when I'm shutting down that there's a problem. The tabs I had open when I shut down are listed in the history but I don't want to have to sort through the last couple weeks of history to find them. I do tend to have a lot of tabs open but most of the time it works fine (except for the daily crashes, after which it restores the tabs).