probably a new site... This happens a lot with fresh sites that google gives good rankings to during the first 2-4 weeks of their existence, but when the free ride ends the sites vanish from the SERPs, at which point you can start doing SEO to them to bring them back. SEO usually means build links to them, but it could also be add more content, improve existing content, improve the on-page SEO signals (site speed, broken links, content quality and relevancy, mobile friendliness, etc...)
And speaking of mobile friendliness, I've heard people say that sites that are not mobile friendly don't even index anymore and if this is true it's one hell of an important ranking factor...
But anyway, that's what I think happened to you. If you're site is not new and had been ranking for months / years before this happened, in this case it's most likely you've been affected by a google update (lots of those happening this year), or you're simply being kicked out of the SERPs by the scumbags at google to be forced to pay them for ads. Many honest sites are being screwed like this, we live in an age of great revelations + great scumbaggery, nearly every company has the mission to destroy as many people as possible....
