Remove the power button board and look at underside of power button. It should have some plastic tabs which give it some elasticity. Where the button touches the power button board you should be able to feel a slight click with your finger when you push on it. This creates enough force together with plastic tabs to push the button out. If there’s anything need to replace, you may just purchase a new one.
If you know few thing about disassembly, you can just go to a local repair spot to get this fixed. Or you can call the technical support at 800-474-6836. If you live outside the US/Canada Region, you may click the following link to get the support number for your region: ++Phone Assist Worldwide++.
Hope this helps.