I am working in SP online. I just realized that for some lists/libraries the search box is not working at all.
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Try redindexing the list: List Settings -> Advanced settings -> Reindex List
If there are multiple lists on same site affected you might want to index whole site: Site Settings -> Search -> Search and Offline Availability
Do note that in SPO it takes some time before reindexing kicks in so it might take some hours or a day until reindexing has full effect.
I fixed this by going to Site Settings > Site and offline availability > changed "Indexing Site Content" from No to Yes, then triggered "Reindex site" from that page.
Then I went to List Settings > Advanced Settings > "Reindex site"
And ten minutes later, the list items were appearing in search results.