Every search engine runs the same three stage pipeline, and a page has to clear all three before a human ever sees it in results. A failure at any stage makes the later stages irrelevant.
Crawl. A bot (Googlebot, Bingbot) fetches your URL over HTTP, following links and reading your sitemap to discover new addresses. If the fetch returns a 500 or the path is blocked in robots.txt, nothing downstream happens.
Index. The engine renders the page, extracts its text, images and structured data, then stores a processed copy in a huge lookup structure keyed by words and phrases. A crawled page is not automatically indexed; the engine may judge it a duplicate or too thin to keep.
Rank. Only at query time does the engine pull the matching indexed pages and order them for that one search. Ranking is computed per query, not a fixed score stapled to your page.
When traffic drops, diagnose in pipeline order. Confirm the page is crawlable, then that it is indexed (a site: search or Search Console), then worry about position. Running rank tactics on a page Google cannot even fetch wastes weeks.