From Top 5 to nowhere, no penalty, no changes, nothing? why?

One of my pages was ranking on some of the most competitive keywords for very long, suddenly few days ago, it disappeared. When I paste URL it shows indexed but I can't find the other, even If I type keyword + domain name then also the page doesn't show up instead other related pages of site shows up.
This happened to many of the keywords but not with every keyword, many pages are still ranking the same but all those were at the top are nowhere now.
What could be the reason, traffic dropped nearly 70%.

might be an update, but we don't know yet (other people have experienced this, as well)
https://www.blackhatworld.com/seo/sudden-huge-google-traffic-drop.1390490/