I updated a lengthy document and suddenly lost my format. I think it was operator error but I don’t know what I did wrong.
We can’t tell for you!
If you’re looking for a Writer user’s guide, download it for free from the official site.
There’s also an excellent book, but I don’t remember its exact title, something like “Do it with Styles”.
I can also recommend RGB-es’s book To Tame a Writer.
There are also numerous YouTube tutorials by “LibreOffice - step by step”.