Blank document with only comments I added

Sounds like the file might be corrupted, but if the comments are still there then it should be possible to export them from that file and then import them back into a fresh copy of it. However, doing that in Reader requires using a script (like this paid-for one I've developed: https://www.try67.com/tool/adobe-reader-import-export-pdf-form-data), but you can use the free version of Acrobat to do it, if it's just a one-time thing. You can also use Acrobat to try and fix the file, by re-creating it (adding the pages from it to a new blank file) or flattening it (printing it to the Adobe PDF virtual printer to create a new copy).

@happy_discoverer9952 can you try to use the "Print to PDF" function. Open the file with your comments in Adobe Reader, and instead of saving, go to File > Print. From the print dialogue box, select "Save as PDF" or "Print to PDF" from the printer dropdown menu?

This creates a new, fresh PDF file that includes both the original document and your comments. This would effectively bypassing any corruption issues with the original file. You can then save this new file and continue your work without fear of losing your notes.