Asus monitor not working as external monitor for notebook

Hi,

i have a notebook lenovo y570 that has a intel 3000 and a nvidia gt 555m card. My external asus vg248 monitor used to work through hdmi, but now it doesnt. I test with another laptop and the monitor works through hdmi... i must have changed some setting in my lenovo. The monitor says no hdmi signal.

I think something is wrong here:

help please!!

See if this helps:

Extending the desktop to the attached monitor with Microsoft Windows

1. Right-click the desktop, and then click Properties.
2. In the Display Properties dialog box, select the Settings tab.
3. Click the Display list and select your external monitor.
   Note: For help identifying the monitors, click Identify.
4. Select Extend my Windows desktop onto this monitor, then click Apply or OK.