Diagnosis
This is almost always a handshake issue between the monitor's DisplayPort and HDMI ports. The U27P2 defaults to DisplayPort input, so if you're plugged into HDMI, it's looking in the wrong place.
How to Fix It
Power on the monitor. Press the joystick button on the back to bring up the OSD. Navigate down to 'Input Select' and manually choose 'HDMI 1' or 'HDMI 2', whichever port you're using. Your signal should pop right up. Once it does, go into the OSD menu again, navigate to 'Setup' -> 'Input Auto Switch' and make sure it's set to 'On' so it finds the signal automatically next time.
If That Didn't Work
Try a different, known-good HDMI cable. The 4K signal requires a high-bandwidth cable (HDMI 2.0 or better). A cheap or old cable might not handle it, causing a persistent 'No Signal'.
Heads Up
If you ever plug a DisplayPort cable in, the monitor will likely default back to that input the next time you power on. Get used to hitting that joystick to manually switch inputs if you're swapping between sources.