iPad freezes and won't turn back on
If you iPad is currently spinning trying to boot and you don’t have access to the power button there isn’t much you can do at the moment. Hopefully allowing you Device to continue to try to boot will drain the batteries. This will allow your device to shut down, then start charging and hopefully it will boot up. Start charging with your computer so you can tell if your computer is recognizing the device while it’s booting. Going to About my Mac > System Profile > USB will show you if your device is registering with your computer. 

If not, and you are comfortable following an ifixit tear down, you could take it apart to see if you can either clean up the power button (it’s probably dirty) or manually press the power-button if the little lever is broken on the button, which you could use for DFU or Recovery. 

Search for your models water sensors, usually in the headphone and charging jack to see if there is water damage.