only when on - power button doesnt work

Hello gents

My power button doesn't work when my device is on.

I know its not a physical problem because when I pull out the battery then put it back in and hold the power button, the device turns on.

It only doesn't work when the device is on.

Tried holding it down for a while but nothing.

My device is not rooted.
I am on 4.0.4

thanks