Diagnosis
No power delivery to the monitor. The primary suspect is the external power brick or its cable connections.
How to Fix It
First, check the physical connections. Unplug the AC power adapter from both the wall and the monitor. Inspect the DC power cable connecting the brick to the monitor. It must be fully seated in the port on the back of the monitor. Reconnect firmly. Next, verify the AC power cable is fully inserted into the power brick. Finally, plug the AC cable into a known-good wall outlet. The monitor should power on without needing the PC.
If That Didn't Work
The external 65W power adapter (model HA65NS1-00) may have failed. This is a common single point of failure. You must test with a known-good, compatible 19.5V DC adapter. Contact Dell Support for a replacement.
Heads Up
Do not attempt to use a power adapter with a different voltage rating. Using an incorrect adapter can permanently damage the monitor's internal power circuitry.