clickpatrick , Try a reset, press and hold the home button. While holding the home button, press and hold the sleep/wake button. Continue holding both buttons for about 10 seconds, until the Apple logo appears.
Indeed if the battery is dead the device is not going to power on. Try charging the battery for a few hours and try, still no go try another compatible charger/cable and charge a few hours, if powers up defective charger. If it does not power up, take the device apart and check to see if battery/charge port connections are all good, check all connection and components on the board for loose/damaged and reseat connections to have good contact. Check to see if battery has a charge with a DVOM should see 3.7 volts more or less (charge no good , probable faulty battery). If battery OK reassemble and try the device. No go, could b bad display or logic MB(mother board). Disassembly and troubleshooting guide links below. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
iPhone 6 Teardown
iPhone 6 Troubleshooting