Is there a way to recover lost sessions

Only if you create and save some backups to Google Drive or a file, it's possible to create them manually in free mode, in paid mode they must be created automatically every day.
Alternatively you can try to revert the whole Chrome Profile directory back in time using your system backup or some other system features (Time-machine on MAC or folder Previous Version on Windows ), but this a complex task actually, with not very guaranteed results, and possible with many new troubles, so basically the only practical option is the backups you make from Tabs Outliner itself, if any.

Talking by experience they are gone. I would have love VLAD if you had made it more obvious that you can use drive to save 2 stages of the tree completely for free. That would have saved me a lot of pain losing all my nodes from up to 2 years. Now I am in the pro version so I am happy being able to use more than one computer and have all my nodes available.

MAYBE adding this info to the quick start help menu.