|Heh, I have been troubleshooting the same issue on my ASUS K55V for a week. Here is how I solved it.|
A coupIe weeks ago, I purchased a small HDTV to watch the Olympics and also use as an external monitor along with a mid-price HMDI cable to connect. Everything worked right off plug and play. But after one week, suddenly, it was gone, no other display was deteceted. Poof!
Like you, I did all the usual stuff suggested on the forums, driver updates/fallbacks downloaded from ASUS, Intel, Nvidia..., checked screen resolution for HD monitor to output in 60hz, I restored Windows system to dates prior to loss of HDMI detection, but no luck.
Finally, even though *everyone* told me all hdmi cables are alike, highly unlikely solution, I purchased more expensive HDMI cables. Both, ULTRA and INVEO HDMI cables solved the problem instantly. In my case, as confirmed by some forum posters, ASUS manufacturer apparently has recessed the hdmi port out of reach of ordinary HDMI cable connectors.
I would encourage you to continue to testing HDMI cables. Regards!, Lydie