How do I unfreeze an iPhone 6 when the reboot doesn't work?

I've got an iPhone 6 that's frozen and doesn't seem to let me onto any of my apps its completely frozen I've tried the on/off button and the home button for 10 seconds but that hasn't seemed to work don't know what else to try

Leave it on charge for several hours then try volume + home for 15 seconds while still on charge.

If that does not work, try a DFU restore via itunes. You will lose all the content.

If you can't get it to a state where itunes recognises it in DFU mode, it's got some form of internal damage. All you can do is take it to an Apple shop and see what they say…

It's also possible it is stuck in DFU mode already, as that leaves the screen blank…
The same sequence to put one in DFU also switches it back to normal, as long as the firmware is OK.