How do I restart my iPad when it is not turning on but without a power button

Ngan2 wrote:

but without a power button

Start here.

iPad Repair - Official Apple Support

If the physical Sleep/Wake (Power) button is broken, your iPad will require repair - as, sooner or later, you will encounter a problem from which you cannot recover without this button being fully functional.

However, if the iPad is otherwise fully functional, you can power-on your iPad simply by connecting the iPad to its Power Adapter. As soon as external power is applied, if the battery has some level of charge, the iPad should simply power-on or restart.