I'm banging my head against the wall trying to figure out why I sometimes cannot get any video to display with this setup. We just purchased about 40 new Mac Minis (late 2014 model with Iris integrated graphics) along with Dell P2317H monitors. We're connecting them via HDMI using the built-in ports, no adapters involved. Of the 28 that I have deployed so far, 6 have had this issue right out of the box. The monitor will detect that the HDMI cable has been connected, but then says no signal coming from device and goes into sleep mode. I've tried SMC and PRAM resets to no avail. A miniDP-VGA adapter and VGA cable work fine and the same monitor/cable will work when connected to my mid-2015 15" Retina MacBook Pro's HDMI cable. I'm also able to netboot the Minis and remote into them and can see that with just the HDMI connected, System Profiler doesn't detect any displays.
I feel like this has to be some sort of EDID issue, as googling indicates similar issues with Minis and certain monitors. The only potential fix I've found involves creating a display override file, which with SIP is a huge pain. I also don't understand why if this is the issue, it's only affecting about 25% of the systems, when they are all setup exactly the same.
This may not be a solution, but what happens if you use the DisplayPort->miniDisplayPort cable that comes in the display box? Depending on the display, this is sometime a better solution anyway because DisplayPort can support certain refresh rates that HDMI doesn't support. And, this would be just as clean of a solution as using HDMI because there wouldn't be any adapter required either. I have a different Dell monitor (UP2716) and once I made this change I haven't had any wake-from-sleep or no-video issues.
@bmarks It looks like getting mDP-DP cable is a perk of the extra few hundred dollars you're spending to get a monitor in the UltraSharp series ;)
@CapU Not sure, but at this point the "solution" has been to swap to a different brand of HDMI cable and possibly restart. Once the Mini and monitor start talking, switch back to the old cable. At this point things seem to work fine, no matter how many power cycles I do on the computer and monitor. Crossing my fingers...