怎样强制重启 iPhone 6

I have an Iphone 6 which just broke in mid use. No GPS signal then freeses. Now it only goes as far as the apple logo. Tried to update and replace from a backup but nothing works. I havent goten it back but service says it needs replacement for 379Euros. Do you have any ideas on how to debug what needs to replaced? Mobo, something else maybe? does anybody know what IOS requires from the hardware to be able boot up? What post tests are there. The phone has always been in a lifeproof nuud case thats really robust and waterproof so I can't understand what could have gone wrong on this...