DIAGNOSIS The USB-C connection isn't establishing a video link. The most common cause is an insufficient USB-C cable or a laptop port not supporting DisplayPort Alt Mode.
FIX First, verify your MacBook model supports video output over USB-C. Then, use the Dell-supplied USB-C cable that came with the monitor. If you lost it, you must use a certified 10 Gbps (or Thunderbolt) USB-C cable that explicitly supports DisplayPort Alt Mode and 90W power delivery. Do not use a generic charging cable.
IF THAT DIDN'T WORK Reset the monitor's settings. Go to Menu > Others > Factory Reset. Power cycle the monitor after the reset. Also, try a different USB-C port on your MacBook.
HEADS UP Some older or budget laptops only support USB-C for data and charging, not video. The U2723QE requires a USB-C source with DisplayPort Alt Mode capability.