Added: On further investigation it seems Google detected some sort of unusual activity on my account, assumed it was a hijack in progress, and disabled everything until I could verify my identity with them and reset the account. Fortunately I was online when it happened and was able to deal with everything immediately. I've since backed up what I didn't already have saved from the blog and added a few additional security measures to protect my content.
I don't think I can describe how upsetting it is to see the last decade of your online writing life vanish as if it never existed. I do have every year saved, but as I was thinking of how much work it would be to recreate the blog I admit, I felt quite discouraged. I might start a mirror blog in private somewhere and start copying the content there in case this happens again. In the meantime, if you have a blog but have not backed up your content, you might want to make an effort, just in case.