Phone won't charge after plugged to the wall
My iPhone won't charge when connected to USB port
Try using a different USB cable or power adapter. That may fix it.
If not, there may be a problem with the lightning connector. Make sure the connector is plugged in. Also, it could be a battery problem.
It's probably Just yr cable, try another lead first. If won't turn on still, just hold power & home button together till Apple
If it is not the battery or the USB cords, there is a charging port device in the phone which is an inexpensive replacement. Call someone who can service iPhones in your area. Unlikely but more serious--a U2 IC chip is bad--which may have to be replaced by Apple.