I went through something similar yesterday. I close my browser with lots of tabs open of things I need to get back to later. Because of some research I was doing, I had over 100 tabs open yesterday. It ends up I had accidentally dragged one tab off the browser into a new window. When I shut down my big window, there was another left, so it forgot all 100+ of my tabs. I'm now trying to recreate them all, relogin to a bunch of them, and remember where I left off. So frustrating.