A laptop and monitor must agree on three things: the physical connection, the signal direction, and the display mode. When any one of those is wrong, the monitor reports no signal even though both devices are on.
Cables and adapters are the weakest link. Not every USB-C port can carry video, and not every HDMI cable supports the resolution and refresh rate being sent. A cable that works elsewhere may still be the wrong type for this specific port.
The laptop also has to decide where to send the picture. By default it may keep the internal display active and ignore the external one. Pressing the projection key combination cycles through duplicate, extend and second-screen-only modes.
Monitors sometimes need their input source selected manually. A monitor with multiple HDMI ports will stay black if it is looking at the wrong port, which looks exactly like a laptop detection failure.