I just started having this exact same problem. I used the monitor for work (via HDMI) yesterday. I switched over to another machine connected via DisplayPort to play some video games last night. Today, the HDMI input doesn't register any input? I tried hooking a mac laptop up via HDMI dongle (i have two and tried both) and that didn't register either.
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I cannot speak to the Mac mDP to HDMI dongles (assuming the Mac has mDP out port?). If possible, you should be testing via a computer HDMI out port --> HDMI to HDMI cable --> S2716DG HDMI in port.
* Turn the monitor off
* Disconnect the monitor power cable from the rear/bottom of the monitor
* If a mechanical monitor power button, press the button in for 30 seconds
* Reconnect the power cable to the monitor. The other end should be plugged directly into the wall socket. No UPS or surge protector
* Reconnect the video cable and retest