How can I restore my contacts on my iPhone XR?

Well, there are many possibilities. I’m not sure where to start guessing with no additional information from you.

One possibility is the contacts are housed on your iPhone. But then it’s hard to imagine how they would all have been deleted.

In this case you could restore your latest iPhone backup to retrieve the contacts, but again this scenario seems unlikely.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

The answer depends completely on where your contacts are housed.