Lost All Past Sessions From Simple Mac Restart

I dearly love your extension and have been considering an upgrade to the paid version. However, I was doing some work today and before I knew it had run out of hard drive/disc space. 

I quickly quit Chrome+Tabs Outliner and restarted my computer, since this has happened to me once or twice before and I find that, unless I do a quick restart, the Finder app slows down and makes it near-impossible to modify the files taking up the biggest space (usually I switch all the ones I'm not currently using/using anymore to "online-only"/"non-local" or "remove download" and that does the trick until I actually have time to meticulously go through everything to clear up more space). 

I've done this before and had no previous issue with the Tabs Outliner extension. Until today. 

For some reason, ALL of my previous sessions/trees/tabs are gone. Just... gone, like the current session that it opens to is the first one ever. 

I'm desperately hoping that there's some way to restore them, or to at least have access to the previously recorded data/history. 

Where have they all gone? Why were they arbitrarily erased from a routine action that I've done at least twice before while using the extension and had no issues with until now? 

I would very much like to restore my past saved sessions--or at least have some record of or access to my previously saved data. I would also love to know what happened/what I may have contributed on my end so I can avoid it in the future. 

Do you have any knowledge regarding this and/or can provide any help? It would be greatly appreciated. 

Thank you, CL