The most involved method to restart a phone without the power button is ADB---Android Debug Bridge. This will only work if the device is powered on already, though.

How to Force Restart an Android Phone Without a Power Button If your Android phone’s power button is broken or not working, you can still force restart it using the following methods: ...

If you want to restart your Android phone without the power button with a screen turned off, do these methods. Method 1: Plug in a charger to your dead-battery Android phone.

There are few ways to turn on a phone with a broken power button, unless you're willing to work to get your hands dirty with developer settings.

If your Android phone is frozen or unresponsive, restart the device by pressing and holding the power button until the phone reboots.