A few ideas:
First, definitely download a back-up of your file if you haven’t already. (Per my previous post: click the File icon next to the file name, and choose Download). When it downloads: how big is the file in question?
After you download, try starting a new file with almost nothing in it and saving it as a new file. See if that works. (It would give me some sense of whether the problem with the other file is its size).
If you aren’t already, try using SketchUp for Web in the Google Chrome web browser.
SketchUp Pro starts with a 30-day free trial, so there’s no harm in trying out the desktop version to see if you have better luck. With a file save problem like this, I’m worried that your file may have been corrupted in some way. Did you download any components from the 3D Warehouse into your model?
Thanks for hanging in there; I’m sorry to hear you lost your progress! If the file saved at all, you can try to recover it in the revision history. (https://help.sketchup.com/en/sketchup-web/saving-opening-and-downloading-models#revision-history)