The Mac uses a system that reminds me of “Plug and play” to determine what display is connected, and what its capabilities are.
To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.
This query is only sent at certain times:
• at startup
• at wake from sleep — so momentarily sleeping and waking your Mac may work
• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go
• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)
if your display does not light up, and 720p resolution is not selectable FOR THAT SPECIFIC DISPLAY (each display will be different) then that display is not telling the Mac it is capable of 720p.