why is my phone not charging

Well, several possibilities come to mind:

• Your USB power source is defective
• Your USB to Lightning cable is defective
• The connector on the bottom of the phone is full of crud (clean it with a toothpick)
• The phone has water damage
• There is a hardware problem with the phone, most likely the charger chip in the phone
• The battery was replaced with the wrong battery
• You used a really, really cheap Lightning cable→Why Counterfeit Lightning Cables Kill iPhones - Motherboard and it killed your phone’s charger chip

Have you tried charging with a wireless charger? Does that work? If it does, then the problem is one of the first four above.