I connected my HP Pavilion dv4-2165dx to my TV via the HDMI slot, and worked perfectly. I disconnected the cable from the notebook when I was done and still got the main LCD display to work. When I restarted the system, the LCD display was gone, just a blank screen with a backlight. The system is booting correctly, I can hear the windows7 startup sounds and even type my password in and access my account. So far I know that:
1. The screen hardware is not damaged, because after some reboots and taking the battery off for an hour I could get the input back with no problem (with windows complaining about being unable to boot and suggesting me to run Startup repair), but got a black screen again after rebooting again.
2. I can connect the HDMI again and get the output on the TV screen, but no main display is detected. If I toggle between displays with fn+f4, only the external HDMI-TV display works.
3. I get no startup screen either. That is, not even the first screen from where I can access the BIOS. But if I connect the HDMI-TV from the beginning I get all that info on the TV.
4. I reinstalled the Graphics driver from HP with no change.
My guess is that somewhat I messed up the default configuration and that the system is only recognizing the external displays as default displays. I haven't been able to modify this from the Control Panel - Display Settings menu, and I don't know where in the registry this default startup options could be.