Why could there be a substitution of the address in the search results?

Question

Search Engine Optimization

The problem is as follows: There is a VPS that hosts the problem site and several other sites. IP address, of course, one.
All sites on the server work in normal http, the main site (problem) works on https.
Until recently, everything worked fine, but a few days ago, the main site (site1) began to display incorrectly in Yandex search results.
The title, description of the site belong to him, everything is correct, but for some reason the site address began to point to a completely different site (site2). Also, the favicon has changed to an icon from another site (site2). Moreover, the address of site2 began to be displayed with https, although site2 works on http.
Both of these sites are located on the same server, but why could such a problem occur?
Everything is fine in the search results of Google.
Recently, no manipulations have been carried out with the server, no settings have been changed, nothing has been touched.

Answer 1

check all the files, everything completely, maybe there was a hacking and infection, I also had a similar one, at first everything was fine, just one file was lying that they didn’t notice, and then one day, he downloaded the rest for himself and the search engines began to see the Chinese site instead of mine.

Answer 2

what site opens on IP?