After you force shutdown your laptop, you may experience that your external monitor gets no video when the laptop starts again. You may notice that any USB devices connected to the monitor are also not recognized by the laptop.
You may have similar experience with monitors that support Dell Power Button Sync (DPBS) function when you force shutdown the system with the monitor power button.
No cause information is available.
Unplug and reconnect USB Type-C cable to the monitor.