Have you tried to reset the search index and see if it works?
On DNN 8, as host user, you can go to the Admin > Search Admin to reset the index and then go to the Host > Schedule page. Look for the Site crawler task and force it to run right away. Browse around the site and wait a few minutes and then retry.
On DNN 9+, as host user, you can go to the Settings > Site Settings to reset the index and then go to the Settings > Scheduler. Look for the Site crawler task and force it to run right away. Browse around the site and wait a few minutes and then retry.