Phone only boots when power button is held down

Hey I've had this phone for a while and at the moment it refuses to turn on. The weird part is It when the power is held down, the screen will turn onto the startup screen it then will proceed through the startup process for as long as the power button is held down and then turn off when either it would normally go to the home screen ,roughly 12 seconds, or when the power button is released any ideas why it will stay on for that extra time if the power button is held down

thank you :)