How do I open this Read-error writer file

I created a writer text file (there is some xml coding in it) and worked on it several times,usually backing up at the end of the day. The one time I didn’t backup for a couple of days, I can’t open the file due to a Read-Error “Format error discovered in the file in sub-document content.xml etc”. I’ve tried solutions I found here or elsewhere but either I’m doing them wrong (quite possible) or they don’t work for some other reason. I’m using Windows 8.1 and LO 5.0.2.2.
Any (simple as poss)step-by-step how to fix instructions would be greatly appreciated !

This seems to be a duplicate of

Solved it. Thanks anyway.

Can you describe the solution.

Can you describe the solution.