If it won't power off by pressing the power button, then you are going to have to either pull the battery (Yes, I know some tablet batteries are not so easy to get to, on some devices. In such a case, make sure that your tablet is not under warranty still before you try opening it up yourself. If it is under warranty, then contact the manufacturer first.) or you are going to have to wait for it to run out of power.
Once is it out of power (if you have to go that route) then let it charge for a short while only and then try turning it on normally or doing the reset. I say only charge it a little as, should it get stuck again, then you won't have to wait as long for it to run out of power. Once it is working correctly, then you can charge it fully.