Start by checking if anything changed recently, like the landing page, the offer, or the ad copy. Then look at your search terms report to see if irrelevant traffic started coming in. Check your conversion tracking to make sure it is still firing correctly, because sometimes a website update breaks the tag. Also look at your Quality Score and whether your ads are still showing for the right keywords. A sudden drop usually has one clear cause once you dig in. What is the first thing you usually check when a campaign goes quiet?