Thanks cdbullyweefan. Now sorted. I'll publicly shame myself now, as I expect I have done something really dumb! When I went in this morning on the desktop version I noticed the unread posts button on the right hand side for the first time ever. I appear to have accidentally checked or unchecked this on the desktop version, hiding all the threads, then gone on on the mobile version, which I don't think has this button, and become very confused.
Conclusion - I am an idiot. Martin was very helpfully looking into this so I have let him know it's all sorted.