Why won't my iPhone 6 turn on after I put it on the charger?

Has the phone failed following some kind of repair or impact? As @benjamen50 says, it is most likely just a dead battery but there could be other reasons why this is happening depending on the history of your device. The more information you give us the better we can help you.

Some ideas...

• The battery died a natural death.
• The battery connection is loose following a fall or impact. You need to verify and re-seat the connector.
• Your charge circuit on the logic board could be defective. This can be caused by using cheap quality or expensive-fake charging block and Lightning wire. The over-voltage protection is not in the phone but in the cable and charging block.