My CPA went up, what do I do?

CPA on one of my Search campaigns jumped almost 40% over the last week and a half. Budget hasn't changed, bidding strategy hasn't changed, I'm not aware of any new competitors. Impressions are steady but conversion rate dropped. Where do I actually start looking, is this a tracking problem, an auction problem, or something on the landing page? Not looking for "it depends," looking for the actual order of checks people run when this happens.