The DNN search should index and search products automatically but if for some reason it's not working in your portal then follow these instructions to re index and fix.
1. Login as admin and go to your DNN site settings -> search
2. You will find a path where dnn stores index files. See the following screen shot.
3. Stop the IIS app pool for the site or reset IIS and delete all files in this folder.
4. Open your site and wait until the Site Crawler scheduler runs again or run it manually from the DNN schedule as follows: