How can I fix my power button at home?

Enter download mode by holding both volume buttons. Now attach your USB (already attached to your computer's USB port) now cancel the download by pressing the volume down button and it will restart your phone. Go to the play store and download a free app called button savior. This will allow you to turn it on or off etc. I understand there are other apps that work similarly,. It's just the one I've used before. If the phone bootloops instead it likely means the switch is shorted and you might have to tear it down and remove the defective switch to achieve anything. (You can try working it a bit or cleaning it while seeking divine intervention) That's a challenge since it properly needs micro soldering and microscopes.. . possible without but pretty darn tough. This is board level repair, I'd advise a good shop. If you've rooted the device and are on a custom ROM you'll also need to download another app called recovery reboot, otherwise you won't be able to enter recovery mode.