Monitor Going into power save mode

Hello, Lately I've been having a problem with my XPS 420 Desktop computer, everything was fine till a day or two ago when I started noticing for some odd reason the monitor goes into power save mode,I could be on the internet for HOURS upon HOURS but the moment I start up a game like World of Warcraft...within a few min's it goes into power save mode (sometimes even when im surfing the internet). I can tell my PC is still running in the backround because I can hear music or game sounds still being played but my screen is black and I cant do anything..moving the mouse hitting buttons on the keyboard using the tower on and off button just turns it off. So the only way I can get rid of it is to restart the computer.

Also It's a 19 inch LCD monitor that CAME with the XPS 420 computer