Factory reset problem

My dell tablet had been stollen and I’ve just gotten it back. But they put a password on it and I have no idea what it is. I’ve tried factory resetting it by the power button and volume down button. Then I choose recovery. But when I do it just takes me to a screen with the dell guy laying down with a red triangle with an exclamation mark above him. I tried holding the volume down button and power button again but it just shuts the tablet off. I have no idea what to do anymore. :(