Here are some of the many things that can cause a page to disappear from the SERPs.
** Another webmaster grabbed your page and published it on his domain and now he is ranking for your keywords instead of you.
** You grabbed another webmaster's page and published it on your site and now that page on your site has disappeared from the SERPs.
** You have built or received links on other websites with anchor text that matches the keywords that you are no longer ranking for. (a page falling from the SERPs for this usually has a number of exact match keyword links)
** Your page has text or an image or other intellectual property on it that belongs to another person and they filed DMCA to Google. Now your page is not ranking.
** Another person has filed a false DMCA to Google that accuses you of taking their content.
** You have accidentally placed a no index for this page on your website.
** A hacker has placed a no index for this page on your website.
** These pages are relatively new and are in-and-out of the SERPs like many new pages do.
** You have done (intentionally or innocently) some sin and now Google has whacked your page.
Usually one of the above will account for a page missing from the SERPs. There are other reasons... but the above usually nail it. Rarely, Google will temporarily lose track of a page (really rare but can happen).