I've seen a number of people have trouble with the HDMI output and Raspberry Pi's. Not on every display, but it seems especally true when using something like an HDMI to VGA Adapter. As far as I can tell this works across every version of the Pi, and every OS (as all of them need the config.txt file). This includes Windows 10 IoT.
If you turn on your Pi for the first time and get no video output, try adding/uncommenting the following two lines.
The first option should force HDMI mode in case it is not detected. Some systems appearently do not send the signal to use this or do not send it fast enough. If it isn't detected, the Pi will not continue trying to display over HDMI and you will not get any video.
The second forces HDMI into DMT (computer monitor) mode. This shouldn't strictly be necessary and should only really help with sound issuse.