If you’ve installed KB4515384 patch, you’ll have to choose between uninstalling it, or living without search:

“Until Microsoft issues a fix, you’re going to have to either choose between Cortana and search causing spikes in your CPU usage, or applying the latest patch and losing search altogether. In our view, we’d rather sacrifice search in order to keep CPU usage down – but it’s a choice we shouldn’t have to make.”

https://www.techradar.com/news/microsoft-tries-to-fix-windows-10-ends-up-breaking-the-start-menu