Usually this will be caused by the graphics driver not properly supporting sleep modes and therefore disabling the ability to detect a connected display when the computer goes to sleep.
Update your graphics drivers from the manufacturer to resolve the issue.
If the issue is still not resolved after updating the drivers, it may be due to an unusually reported display ID (EDID). You can usually force the detection of a monitor within the graphics driver control panel.
However if this is unavailable, you can use the Windows Resolution window to detect devices while the monitor is disconnected, then select the display which will report “Another Display Not Connected”, then under Multiple Displays, you can drop down and select “Try to connect anyway” which will broadcast a signal to the monitor over the port type you have selected (the signal will not be interactive), then connect the display which should interpret the signals sent to it properly.
The forced resolution will override detection of the screen and should persist through sleep.