my iphone7 plus just shut down after charging it, how can i fix this

this is the first time my iphone 7 plus had shut down on me, i put it to charge just before going to bed like after 10pm and when i woke up at 6am, the phone wasn't coming on, i thought it might of bin a problem where it wasnt charging, so i put it back on charge for another 2 hours while at work and nothing still and its not connecting to itunes and its still not coming on and its now 1:15pm.

how do i fix this