To troubleshoot search not returning any results in SharePoint, check the following:

1. Check and verify that the status of the search services is 'Online' in Central Administration.
2. Check if there are sufficient permissions to use the search service.
3. Check if the crawl has actually happened and content sources/websites have been indexed.
4. Try re-indexing the content sources/websites.
5. Check if searchable columns have been created in the list.