Anyway, as I got in from work early I thought I'd pull a cheeky half off to see how it looks, smells and tastes (I've been patiently waiting for quite some time now!). My problem is, as soon as I opened the tap air rushed IN throught the tap, bubbling through the ale; I take that to mean a vacuum must have been created somehow.

I got a couple of inches out into a glass (where it tastes alright but is obviously a little cloudy) but now I'm unsure what to do next. I was thinking I should whack some CO2 in quick smart to blanket the brew, and leave it to settle again before attempting to draw some more off.
Does this sound about right? And can anyone suggest how this could have happened? I think it must have been the extreme cold, but assumed bringing it in overnight would've have let any gas and fluid expand a bit. Guess not, eh?
By the way it's a budget keg with a brand new S30 on which looks well seated etc.
Cheers,
Sam