Connect a genuine Apple charger and genuine Apple lightening cable to the phone. It should 'prompt to boot'. If nothing happens, then there are a lot of possibilities.
Check the battery connection on the main-board. The battery has a 'gas gauge' circuit. if the phone can't see the gas gauge circuit in the battery, the battery connector may not be seated properly at the main-board and the phone won't boot.
The lightening connector in the phone may be damaged. You may need to replace the lightening connector. Try flipping the lightening cable around 180 degrees and plug it back in. The phone should prompt to boot.
After that has been checked, and the phone does not boot, then some advanced troubleshooting may need to be done. Check with iPadRehab forums, and maybe get a quote for repair from the main WEB site for iPadRehab. Good luck!