Monitor not working

I bought a monitor from them a NEW monitor but the one on display as they had no more stock, When I connected the monitor it keep on going off while working on it and when the laptop go on to sleep mode the monitor go off but it does not come back on with the laptop when i move the mouse and then i need to unplug it from the laptop and back in i informed the manager but he did not reply as i said i am taking it back and want my money back because clearly there is something wrong with it . But i was just ignored I need some action here as i am not prepared to pay that money for something NEW that is not working as it should