Ubuntu doesn't detect hdmi connection

Solution 1:

I ran into this problem a while ago too, and for me the problem was two-fold:

My on-board Intel graphics doesn't recognize my HDMI port; only the internal laptop screen and the VGA port.

Solution:
- Turn on the nVidia card in my BIOS.
  (Note: Hybrid doesn't work: It needs to be Intel or NVidia)
- Install the nVidia drivers
if the HDMI cable is a good one, it needs to be firmly plugged into the port.

Solution:

Push the cable really hard into the slot.