If DNN 4.X search is not working follow these steps

1. Changed the contents of site
2. Run this script
   truncate table {databaseOwner}{objectQualifier}SearchItemWordPositionDELETE {databaseOwner}{objectQualifier}SearchItemWordDELETE {databaseOwner}{objectQualifier}SearchWordDELETE {databaseOwner}{objectQualifier}SearchItem
3. Then under host menu selected "Search Admin " and clicked on " Re-Index Content" link.
   And after that new contents starts coming in search.

If you want that DNN search engine automatically index contents when there is any change in site content then "scheduler for search indexing must be running" .