Are you using an Apple original charger and Lightning cable? Has the device been dropped or otherwise damaged, came into contact with water or was previously repaired? While devices can spontaneously fail, issues usually crop up after situations like those outlined above.
If data retention is not an issue, you can also try to do a full [https://www.theiphonewiki.com/wiki/DFU_Mode|DFU Restore]. You could try to use some third party software tools, like 3uTools or ReiBoot to see if the device can be forced into or out of recovery mode.
Otherwise, there may be an issue on the logic board that is impeding the device from completing it's boot process. This would require help from an experienced micro-solderer.