My websites suddenly disappear from search results without any warning

See, I have multiple sites hosted on my personal VPS. They were running smooth, getting decent traffic and making progress but suddenly without any messages or warning, they disappeared from search results. It's been few weeks like this, but I checked few websites are still getting same traffic as before despite not showing on search results. I have no idea why this is happening nor do I know what measures to take to solve it as I am not aware of the reasons. Is it my server or what? (These websites are built on WordPress) Any help is appreciated.