Hello KB55, please check the following things:
Check your graphics driver version and make sure you are using the correct version from this page (64 or 32 bit)
Try different HDMI cables, if you can; be sure they are HDMI 1.4 cables.
If you are uisng any display adapter/dongle, try without.
Check the steps listed in the Multiple Display FAQ at http://www.intel.com/support/graphics/sb/CS-031040.htm.
Check the troubleshooting steps listed on the HDMI FAQ athttp://www.intel.com/support/graphics/sb/CS-034397.htm.