How do you restart your iPhone if your lock button is broken

Wait till your iPhone dies

It’s the simplest step and way. Just let your iPhone discharge until it turns off. Then, you can recharge it and it should turn on when it reaches a high enough battery level. Although not a very good solution but at least it can shut down your iPhone and restart it without worrying about the broken power button.

Reset Network Settings

If you dare, you can reset the device’s network settings which will restart it but will obviously clear your network settings. This is a better idea than draining the battery but it certainly need another setup of your iPhone’s network settings once it’s booted up.

Use Assistive Touch

You can use Assistive Touch (Settings > Accessibility) to lock and turn off the phone. In case you’re not familiar with this feature, this is a good way to bypass all the physical buttons on your iPhone. If your power button is broken, or your home button, or even the volume knobs, the assistive touch is your friend.