I normally use VNC to connect to my Pi so I have not used the HDMI out in the past couple of weeks. When I first set up my Pi I did it using the same display and HDMI cable I am trying to use now (and that successfully worked with my Apple TV).
For some reason when my Pi boots up I see all of the boot up screens (the Raspberry Pi logo, etc...) But once it gets past that it stops displaying anything at all. I am still able to connect to it and use it through VNC but I'm not sure why the HDMI just stops working halfway through the boot up. I have tried swapping out the cable and rebooting several times. Any insight is greatly appreciated.