First, yes, you're in the right area. This is where the calibre gurus can be found. I'm not one of them, but I'll try to toss a few ideas your way until someone gooier comes along.
I just checked with a couple of books, and I can still download metadata. So whatever the problem is, at least it's not a generalized calibre problem. That means you can probably fix it yourself. Well, either that or you're totally hosed. One of the two.
Since it was working yesterday and isn't working today, *something* changed, somewhere in the world. Have you changed anything on your machine at all, even something you don't think could possibly be related to calibre? Is anything different with your home network, your Internet connection, or something your ISP is doing? There's something different between the working state and the non-working state; it's just a matter of pinning down what it is. Hopefully not the phase of the moon.