All pages pulling up with 404 error

Sometime yesterday, my WordPress site appears to have broken. It has been getting 100-400 hits per day and I got nearly none – I think because none of the pages (other than the home page) are accessible any more!

So – what works – the admin panel, and the home page. The search function pulls the correct listing (and recognizes when there are more than a page of posts) but the link to the second page of results fails, as does clicking on any posts. Also, the category and archive widgets links as well as the tags links all bring up the 404 error page.

I usually edit in Windows Live Writer, and I was able to search back further than usual (usually it fails to pull up >100 posts and that actually worked today) so I suspect the database is intact. I generally run auto updates on my installed plugins, so I suspect a plugin problem but I am not sure where to start looking for it.

Also, I noticed that when I tried to edit a post in the WordPress editor, that in Visual view, I could not see any text (just white letters on white background with red spelling squiggles).

Any advice would be much appreciated.