A similar thing just happened to me. I had a kernel panic while the computer was asleep (apparently an instance of this problem). When the computer came back up, Orion had no tabs open, and the “Reopen All Windows From Last Session” menu option was grayed out.
I have to say, my list of open tabs is very important data to me, to the point that this bug outweighs the nice parts of using Orion. It is possible to go through my history and reopen the pages that I think I had open before, but this is time-consuming and I’m not always sure which pages these were.