Have you tried opening it in WordPad (not Word)? Maybe you could try saving it as any other format first? Maybe you could save it as an .RTF? Maybe you could try opening it in Google Docs. I suspect that is how someone converted it to a .DOC
in the first place.
Try converting it to any other format successfully and try a variety of tools besides Word. Then, hopefully, you can convert it eventually back to DOCX.
Good luck!
Best wishes, Davin Mickelson