I was having this problem and in fact found this post while searching for a solution. I could see all my content in the admin side, and could see my theme and sidebar in the browser, but it only showed 404 messages where the content should be.
BUT! While seeking an answer to Chip’s question, I edited a page, clicked Preview Changes and could see the page fine. Something there reset the whole thing because now my site is working fine.
Weird, but true…