First, establish what actually moved
Spend, conversions, conversion rate, AOV, CPC — which of them changed? A drop in revenue with stable conversion rate is a traffic problem. Stable traffic with falling conversion rate is a site or offer problem.
Then check the boring causes
Tracking, disapprovals, stock, a change in the change history, a competitor in auction insights.
Then compare like with like
Against the same month last year, not last month. Seasonality explains more bad months than anything else.
Then change one thing
And wait long enough to read it. Every account we inherit in trouble has a change history that looks like panic.