macOS/iMac - LG UltraFine controls
Summary

Shutdown your device and completely take the plug out of the socket, to kill its power supply;
Disconnect the power supply of your LG UltraFine monitor as well, by pulling the plug out of socket;
Reconnect(/Power on) everything after ~15 seconds;
The controls should come back to life.

Fully unplugging resets a few things in the UltraFine monitor as well as in the iMac.

Detailed
I spoke with Apple Support and the representative tried to help me solve this problem, but to no avail.
The only thing that session managed to get done was have my iMac restarted in Safe Mode. It did not solve the problem, but in the process my UltraFine's display's orientation changed. (I use it in portrait; It got reset to landscape.)
Since it changed something, try that first.

Next, while moving my desk I had to disconnect my UltraFine and iMac. When I plugged them back to another place, I started the iMac first without the display connected to it. Then I plugged in the power cord of UltraFine. And then I plugged the display's cable to iMac.
I understood that disconnecting an iMac off its power supply for ~15 seconds is how you reset its SMC. I think when I disconnected my machine to move it, I accidentally reset the SMC and that fixed the problem.