what can i do to restart an iphone se without the power button working?

Here are some ways to restart your phone In settings, go to "General">"Accessibility" and turn AssistiveTouch on. You will have a little square with a circle appear on your screen. tap that to open it up, and there will be a lock screen button. Press and hold this, and just like with the lock button, the "slide to power off" will appear..

If the power button is broken and it does not power on when you plug it in, there really is no option left but to take it in to Apple and get it evaluated. Odds are you’re going to have to pay for a replacement unless the power button broke due to a defect.

You‘ve already discovered that the only two possible methods to turn on any iOS device are not working. There is no other option but repair or replace.