What Happens if They Refuse?

So the latest gambit from May will see her bring her deal back to the Commons on the 12th of March. If it is voted down, as it surely must, there will then be two votes on the 13th - one to leave with No Deal, and a second to authorise a short extension to Article 50.

So far, so good.

But what happens if, as is very likely, the Commons vote against all of those options?

(Oh, and for that matter, what if the EU refuse to grant a short extension, instead insisting on 2 years? Or they put a hefty price tag on an extension, as they well might?)