DIAGNOSIS The USB-C port is delivering video data but not enough (or any) power. This monitor's USB-C port provides up to 65W of power delivery, but some devices are picky about the negotiation.
FIX Start with the basics. Unplug the USB-C cable from both the monitor and your MacBook, wait 10 seconds, and reconnect firmly. Ensure you're using a certified Thunderbolt 3 or USB-C cable that supports power delivery and data, not a charge-only cable. Check your MacBook's System Information under Power to see if it recognizes a charger.
IF THAT DIDN'T WORK The monitor might be in a weird power state. Unplug the monitor's power cord from the wall for a full minute to perform a hard reset. This often clears temporary glitches in the USB-C controller. Plug it back in and try again.
HEADS UP The 65W is enough to charge a 13-inch MacBook Pro but will only slow-charge or maintain a 16-inch model under heavy load. It's normal for a high-power laptop to still drain slowly if you're pushing the CPU and GPU.