Solitaire game disappeared from my iPhone

The iPhone does not include a solitaire game by default, so you had to download it from the App Store at some point, as such you can always re-download it from there or from your purchase history.

click here ➜ Redownload apps and games from Apple - Apple Support

In the event it was just moved from the Home Screen into the App Library, you can look for it there and drag it back.

click here ➜ Organize the Home Screen and App Library on your iPhone - Apple Support