Search engines use links in order to crawl your site. So if there is a page that is not getting any links the search engine cannot find this page. There alternative possibilities in how a page get a reference so that it also be found by Google, such as redirects or sitemaps. Still it is better to make sure that users can access important pages as easy as possible.
So one option in order to make your page better accessible is to link to its content from pages of your domain.