Press the sleep/wake button and hold until the slider appears. Drag the slider, and turn the phone off. After a couple of seconds, do the same, to turn the phone.
A reset is usually recommended when the restart won’t work. To reset your device, press and hold simultaneously home button and the sleep/wake button for a couple of seconds, until you see the Apple logo on the screen.
If there is a new iOS available, install it, and you can do it wirelessly without iTunes. Connect to a Wi-Fi network, and plug your iPhone 6 to a power source. Next, click on Settings, General, and choose Software update.
This can help as well. Simply let the battery drain completely, until the phone turns off. Charge it, and once it’s charged, turn it on. Hopefully, the problem with the indicator won’t be there.
Back up your files to iCloud first. Tap on Settings > General > Reset > Erase all Content and Settings. It usually takes several minutes, and when the task is finished, set up the iPhone 6 as a new device.