Check in this order: billing (an expired or declined card stops delivery silently. The account looks active while nothing serves), budget ('limited by budget' means your daily cap ran out early; ads resume tomorrow), status (a paused campaign, a disapproved ad, or a policy flag), and finally rank (bids too low for the current auction).
The right diagnostic tool is built in: Ad Preview & Diagnosis (in Google Ads tools) shows whether your ad serves for a given search and tells you why not. Without you googling yourself and polluting your own stats.
We've lived the sneaky version: a billing hiccup plus an automated policy flag can stack, so fixing one and seeing nothing change doesn't mean you fixed the wrong thing. Clear them all, then re-check preview.