1. First of all, never work 3 hours without saving your work. Save every time you've made an important change or progress in your work.
2. Do you know that when you save a
Scene that action not necessarily saves the drawings created in the scene?
That happens because each
Level has its own file in the disc (which is different from the Scene file), and must be saved too.
The Save All command (in the File menu) saves both Scene+Levels used. But if you just use the Save Scene command and Quit, you never really saved the Levels, so the next time you open the scene, the drawings will not be there (because they don't exist).