Hi! Force your iphone to start. To do this, press Sleep/Wake and Home buttons and hold them for at least 10 seconds.
Try to reset your device. This won't delete your data stored on this device.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
If this does not help, restore your device to factory settings. Note that this will erase the data from your device. Take a look this Apple Support article:
Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.
As a final step, try the DFU mode:
- Connect your device to your computer using a USB cable.
- Turn off the device.
- Press and hold the Power button for 3 seconds
- Press the Home button while holding the Power button and hold both for 10 seconds.
- Release the Power button, but KEEP HOLDING the Home button.
- After about 15 seconds, iTunes will tell you it has detected a device in Recovery Mode.
- Restore your device
More info here: https://www.theiphonewiki.com/wiki/DFU_Mode
If these steps don't work, contact Apple and ask for help.
https://www.apple.com/support/iphone/contact/