TB16, Dell Precision Dual USB-C Thunderbolt Dock: TB18DC, and WD15 Type-C docking Stations are designed to operate in AC power only. AC power loss event makes all devices on the docking station fail to function. Even after the AC power is restored, the dock will still not function correctly as the dock must negotiate a power contract with the system Type-C port. This behavior is evident when there is a power loss but also if AC is applied to the Dock, after being removed, whilst it is plugged into the Dell Portable system.
Parts or systems specific to the Issue:
Unplug the Type-C cable and then plug it in again. Sometimes of power loss you may also need to replug the AC power cable.
The dock is functioning as designed, and the User Guide for the products contains information regarding this behavior.