Step 1: Verify that your page or site is missing
It sounds obvious, but first verify that your page or site is actually missing from Google's index. Many people assume that they are not on Google, when in fact their page simply appears very low in Search results.
Turn off safe search, which might be filtering your results.
Search Google for your site or page:
For a missing site: Do a site search with the syntax site:your_domain_name
Examples: site:example.com or site:example.com/petstore
For a missing page: Search Google for the full URL of your page.