Thanks for the reply, yes I checked the conflicts but it wasn't there either. It seems my joplin had a bug being stuck on fetching resources (I'm using one drive). Any new notes could not be edited whilst it was stuck. I managed to get it working again finally after syncing on my iphone, and eventually after restarting my laptop and joplin a few times managed to free it from being stuck (it didn't get stuck on my phone). So I did lose the note in the end. But it seems to be working ok now.
But it makes me think that to be safe I'll use an external editor for important notes which at least creates an offline backup as you edit.