Something like this happened to me a couple months ago. It’s only happened once, and without any clear reason. I’m suspecting it might have gotten a bit of a power spike on an old piece of gear (Yamaha TG-33) I had connected via MIDI and was powering up while Octatrack was booting. But that’s just a wild suspicion.
I try to save more frequently now. 
It’s mildly frustrating on a Mk1 as it’s a few menus away. The Mk2 has a dedicated ‘Save Proj’ (FUNC + PROJ) shortcut. (Edit to add: it’s like remembering to save parts often as I’ve accidentally hit the ‘reload part’ key combo when meaning to get the contextual menu)
I also try and ensure the Octatrack is fully booted before anything connected to it is powered up; or wait for connected things to be powered up before powering Octatrack.