I think this is a bug that people have noticed over the last few days. Staff were working on it yesterday but it might not be fixed yet.
Have you tried turning off the visual editor, to see if that makes a difference? You can do that in Dashboard > Users > Your Profile. It might also be worth clearing your browser cache. And in the other thread raincoaster suggested highlighting the entire post, copying it, and then, while it is still highlighted, clicking Publish; it's worked in the past.
If it still doesn't work, send an email to staff when support opens to let them know you're still having a problem; you'll need to provide details of exactly what's happening so they can help you.