Alienware M11x HDMI port not working

My girlfriend got her Alienware M11x R2 about 6 months ago

Spec: 

CPU i7

Ram 8gb

HDD 320 gb

She has been using the HDMI port to connect an external monitor at home, but now it's not working anymore.

I tried to use different cable on different monitor, and they seem to have the same problem, it' displayed " no signal" and the laptop can't detect the display also.

It works when I connect on the Display Port. 

I don't know what happen with it, so can anyone help me with this.