Why won’t my iPhone XR turn on?

See this support article:--> If your iPhone won't turn on or is frozen - Apple Support

If the issue persists, it might be a different (hardware?) problem, and I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

It may be toast, but start here:-

If your iPhone won't turn on or is frozen

No idea. See if this helps: https://support.apple.com/en-gb/HT201412