Nine times out of ten it's one of five things: the daily budget is exhausted early, bids are below the auction's going rate, the ads are still 'Under review', the campaign is limited by ad-schedule or location settings, or — if it stopped suddenly — a billing or policy issue flagged in the account's top bar.
Check in that order. Then know this: searching for your own ad is unreliable — Google deliberately stops showing ads to people who never click them, and your location/history skew results. Use the Ad Preview tool inside Google Ads instead of live searches.
If the campaign uses Smart Bidding and recently changed conversion settings, expect a learning-phase lull of up to a week — the algorithm stops bidding when its signal disappears.