Hi dar80,
I believe it's a Software issue. Try to restore your iPhone 6 Plus to resolve the issue. If the problem persist, you may wish to head down your nearest Apple Authorised Service Provider.
(Disclaimer: QCD/A.Lab will restore for you to determine whether its a Software/Hardware issue before providing the correct resolution. The only best alternative is to save all your required information and restore it yourself.)
Do note that any physical damage doesn't cover under Apple Limited Warranty. Any form of Beyond Economic Repair (BER) will void your warranty too.
Cheers.