It is possible to create a custom rule for site navigation which prevents you from accessing the Site Navigation page. You can recover your access by creating a quick link to the page in your site and deleting the rule.
Log into a site where you have access to Manage Site Navigation.
Go to Manage Site Navigation.
Right-click on the link and select Properties to get the URL.
Navigate back to the site where you are missing the link.
Paste the link in the home page or a dashboard message.
You will be able to access the Manage Site Navigation page via this link.
Delete the rule which prevented you from seeing the page and adjust the page securities as necessary.
Delete the homepage link or dashboard message that you used to get access to the page.
Please contact Customer Support if you need more information or additional assistance.