It is better to enter into recovery mode or dfu mode
Here is how to enter dfu or recovery mode. this can solve many problem . But it will delete all data on your iphone or ipod.
Enter iphone recovery mode for that press and hold volume button for iphone 7 and for other iphone like iphone 6 press and hold home button at the same time keep iphone connected to computer and itunes is opened. Now on iphone you will see connect your iphone to itunes.
Now under itunes it show that iphone ipod is detected and it is in recover mode.
Here is more way to troubleshoot ipad iphone wont turn on.
For that first connect your iphone to computer via usb cable install latest itunes on your computer. Now press and hold sleep wake button or( in iphone 7 press and hold on off button) for 3 to 5 second and volume button for 5 second. Now your iphone screen goes black and on your itunes you will get pop up message that itunes is detected iphone, ipod or ipad in recovery mode. Now restore your iphone or ipod.
If you get apple logo on iphone screen or connect to itune symbol on iphone then you have press the button long time repeat it again.