This article explains and provides a solution for why the message "No Computer Signal" may appear in sleep mode.

I have an Apple Studio Display 15" LCD and when my computer goes into sleep mode the Apple Studio Display does not go into sleep mode. Instead a message saying "No Computer Signal" is displayed.

Solution :

The Apple Studio Display should go into sleep mode when you select sleep or when Energy Saver is set to put the system or display to sleep. If you see the message "No Computer Signal" remain on the screen, then the Apple Studio Display needs to be reset to clear a value from its internal memory. To do this, press the reset button on the front of the Apple Studio Display (far left button with the triangle icon) until the LED starts to flash.