If Nokia 5800 is dead due to power switch then you can check it with a simple method - plug in Nokia charger in Nokia 5800 and look at the screen carefully if set is not dead then it will start charging but will not become switch on.
some time we got charger not supported problem in Nokia 5800 so to solve this problem. we have to change a 47k resistance that is indicated in diagram given bellow.
Nokia 5800 charging ok button no power on problem can be due to power switch button. So please remove the old power switch button and replace with new one. If button prints are also damaged then you can see this diagram and make this jumper as shown in diagram given in this post. It will help you to solve power problem in nokia 5800 even in this situation while power button switch prints are damage.