Numbers sheet reported as locked and not owned

Hello brains trust. I've been working on a numbers sheet for a couple of weeks. I opened it today and am told that the sheet is locked, and also that I don't own the document so I can't unlock it. Nobody else uses my laptop, so this is literally impossible. Then, after trying to find a solution in this community from previous posts it has just magically unlocked itself and is now editable. I would love to know what is going on with my Numbers sheet. It's a total time waster and I am keen to avoid it happening again in the future. Is there some way of preventing spurious warnings and blockings? TIA