Why is my lock button not working?

[AC Question]

I have had my s4 for about 18 months with no issues other than a swollen battery. But over the last couple of days my lock button has been playing up. Usually one single press of the off button locks the screen and when you press and hold this button the option menu for turning off the phone/flight mode/restart comes up. However now when I go to lock the phone, this menu comes up regardless and within a few seconds the phone has turned itself off. Was a bit annoying anyway but now I cannot get my phone to turn back on at all. Have tried taking out the battery and putting it back in but all the phone does is start to turn on (without me even pressing the on button) and then turns straight back off again before it gets the whole way. Can anyone help??