You are probably going to need to Restore the iPhone Firmware. But first, connect the iPhone with Wall Charger for 20 minutes or so. It may restart itself and be fine, if so done. If nothing, then, keep on Wall Charger, and Reset, hold both Home and Power Buttons until the iPhone begins to restart. If it does and is fine then done. If nothing, with iTunes open connect iPhone to your computer. Hold both Home and Power Buttons until the iPhone is detected in Recovery Mode, usually takes about 20 seconds. Restore the Firmware with iTunes. If this does not work try DFU Mode Restore, with iTunes open and iPhone connected to computer, hold both Home and Power buttons for exactly 10 seconds, then release Power button, keep holding Home button until iTunes recognizes iPhone in Recovery Mode, about 10-20 seconds more time. If the iPhone screen is black, blank and iTunes says Recovery Mode, then this is actually DFU Mode. Restore the Firmware.