My iphone 6S plus has frozen with the iphone logo showing and have tried to get it to reboot but it won’t

We don't know what suggestions that you have already tried, but if you take the phone to the Apple Store or an authorized Apple Service shop, the first thing that they will do is perform a Forced Restart. If you have not already tried this, it's worth a shot.

Restart your iPhone - Apple Support

There are only 2 results when you perform a Forced Restart

1) The phone returns to normal operation.....or.....

2) The phone has a hardware problem that will need attention at the Apple Store or authorized Apple Service shop

Did you try the suggestions in these articles?

If your iPhone won't turn on or is frozen - Apple Support

If your iOS or iPadOS device restarts or displays the Apple logo or a spinning gear unexpectedly - Apple Support

If they do not work then follow the advice at the end of the documents about contacting Apple.