There are few methods which you help you in restarting if your Android device power button is not working:

Have a luck trying this Option If your Android device is OFF

1. Connect your smartphone phone with charger and wait for few seconds.

2. Try Hard Resetting Using Home & Volume button.

If your smartphone is on then you just install "Power Button to Volume Button" or "Oneclick reboot" app from play store and have fun using your phone.

Source : http://www.indabaa.com/how-to-restart-android-phone-without-using-power-button/

There are few methods which you help you in restarting if your Android device power button is not working:

Have a luck trying this Option If your Android device is OFF

1. Connect your smartphone phone with charger and wait for few seconds.

2. Try Hard Resetting Using Home & Volume button.

If your smartphone is on then you just install "Power Button to Volume Button" or "Oneclick reboot" app from play store and have fun using your phone.

Source : http://www.indabaa.com/how-to-restart-android-phone-without-using-power-button/

Disclosure: I'm the author of the linked article.