There’s an error going around WP.com right now. Lots of people have reported it in about a half-dozen threads.
Report your issue to staff directly using Contact Support on the 24/7 Support link at the bottom of the page. And in the meantime, either use an offline blog editor like Blogdesk which always saves a copy of your work, or just wait it out. I always Copy All before hitting “Publish” anyway; saves SO much heartache, because yeah, you have to re-type the title and add categories, but you can just paste in the contents.