Work through this in order. Most drops are explained in the first three steps, and changing bids before you have looked usually makes the next week unreadable too.
1. Did the tracking break?
A theme update, a new app, a consent banner change. If conversions fell off a cliff rather than sloping, suspect measurement before performance. Compare against your actual store orders.
2. Did something get disapproved?
Check Merchant Center. A feed error can remove a large share of your catalogue overnight with no email that anyone reads.
3. Did you change something recently?
Change history, last 30 days. A budget jump, a target tightened, a structure edit — any of them can put the account back into learning, where numbers are unreliable by definition.
4. Did the mix shift?
If brand traffic fell because another channel paused, blended ROAS drops without anything in Google changing. Check brand versus non-brand separately.
5. Is it seasonal or competitive?
Compare against the same period last year rather than last month. Rising CPCs with stable conversion rate usually means more competition, not a broken account.
6. Only now, bids and targets
And change one thing, then wait. Changing three things at once guarantees you will not know which mattered.
Every panic-driven account we have taken over had the same signature: many changes, close together, each one justified by data produced during the previous change.