When a page on a website has no internal links pointing to it we call it an 'orphaned page'.
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This is sometimes commonplace across other marketing disciplines, where businesses will create separate landing pages that cannot be accessed by navigating through the website. They can sometimes be called "landing pages".
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And they can perform within the search engines, as long as Google knows about their existence - most commonly through their inclusion on a sitemap file.
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However, they do not perform as well as they possibly could, compared to when they have links pointing to them from other pages. Links pass on SEO value, you see.
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We've found in the past that pages like this, despite doing well initially, do tend to drop down in SEO over time. Google see's them as 'orphaned' and less important.
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So our recommendation would be - if the page is important from an SEO perspective - to create links to it from elsewhere on the site, so it can be found by the search engine spiders through internal linking.
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Hope this helps!