You've probably went down the list, but lets just review.
-Check the end that plugs into the device, look at the contacts (gold/silverish) and see if it's discolored, blue/green like. If so then the cable got wet and may or may not work.
-Ensure it's not damage, wires coming out etc.
-make sure the power brick is working fine and/or test on another USB out (i.e. PC/Laptop)
-Test another good charging cable, just to make sure. If the cable you are using isn't an original apple cable then be cautious as it may have some side effects to the charging ability of the phone.
Inspect the charging port in the device.
- Look for similar discoloration if possible (bright light and magnifying glass).
- Brush out (not push in) any dirt/ debris from the port (typically you will here a slight "click" sound when you insert the charging cable.
See if hard restart wakes up. Power Button /wake button and Vol Down until apple logo appears.
If issue isn't resolved - check inside the sim tray to see if the white sticker was triggered (will turn red when water has entered the unit)
If none of these solve the issue then we need to assume it's hardware related.
If you noticed that when you plug the phone in the charger and it gets really warm on the back, then possible board issue and the you will need to see a board specialist (sorry I'm not one, but we have some amazing people on here that may be able to assist, or you can see your local shop).
The other options, that seems to work with us before ruling the board is checking the batteries condition. We shock the battery and see if it's going to recharge while charging. This, of course, is done at a shop. Possibly your battery has gone bad.
Good luck and hope this helps. Keep us in the loop