HDMI as 2nd Monitor

http://www.tomshardware.com/forum/280306-33-geforce-spdif-hdmi
Are you hot plugging in the HDMI cable with the PC turned on? If so, turn off the PC or have it go to sleep and then connect the HDMI cable. Then turn the computer back on. Make sure to manually enable the HDTV in the NVIDIA Control Panel if it is not automatically enabled. See if the HDTV appears in the list of monitors detected by the drivers. Let me know what your results are.
https://forums.geforce.com/default/topic/459307/hdmi-output-on-gts-250/