The site is working, but status code: 404 not found, where is the reason for 404?

Question

DLE

Subject - there is a site gos1.ru - search engines and Google Word stopped seeing it, Yandex webmaster also does not see it and ignores the metric, the reason is that the url is not found by bots.
I ask for help, is there any way to deal with this problem without reinstalling the engine?
I am attaching screenshots.

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Answer 1

The site gives a page with a 404 code, you need to look where you have sinned

Answer 2

What has been done to the engine lately? if so, then you need to undo the edits and see what and how, if not, then you need to look towards nginx (and apache if available) and support usually helps with similar problems and the site gives a 301 redirect https://webmaster.yandex. ru/tools/server-response/... due to ssl