Turn off the tablet completely - not just blanking the screen.
Press the power button until it starts to power up. Release the power button.
Now wait a few seconds then press the power button again. You might have to experiment to find how long to wait. Does a recovery screen appear?
Alternatively, with the tablet turned off completely, press and hold the power button and keep holding while it powers up to see if it produces a recovery screen.
I have seen both these mentioned for other tablets, so hopefully oe will work for you.