Some plugins, in particular Find Duplicates, can switch calibre's search from showing only matched books to showing all the books with matched books highlighted.
To switch it back, go to Preferences / Searching and uncheck the "Highlight search results" box. See the image below for details.
While you are there and if you wish, you can add a button to calibre's search bar to make it easy to switch between the two modes. The above image shows you where to make this change.
The new button will appear to the right of the search bar (after the Go and clear buttons), it will look like this:
- will only show books that match search criteria
or
- will show all books with matching books highlighted.
Adapted from chaley's sticky in Library Management