Perfectly fine file become "corrupted"

Just finished transcribing a choral piece for my kids to work with the piano part and it was an absolute beast. I have opened it so many times before and it was fine, now it will not let me open and it says its corrupt. I changed nothing. I put so many hours of work into this piano part I really don't have time to do it all again!
Please tell me if you can help me! It is attached below.