Diagnosis
The monitor is powered on but the LCD screen backlight or the video feed has failed. The most common culprit is the monitor itself entering a weird sleep mode or the connection to the camera being lost.
How to Fix It
First, try a hard reset. Unplug the monitor from its charger and also unplug the camera's power adapter from the wall. Wait a full 60 seconds. Plug the camera back in first, wait for its status light to settle, then plug the monitor back in. This forces a full reconnection. If the screen lights up but shows 'No Signal,' you need to re-pair them—press and hold the 'Pair' button on the back of the camera until the light blinks, then do the same on the monitor.
If That Didn't Work
The issue might be with the monitor's internal battery or screen. Try plugging it into a different, known-good USB charger and cable. If it works on a different charger, your original charger is dying. If it's still black on a good charger, the monitor's screen or mainboard is likely faulty.
Heads Up
If you have to send it in, Babysense customer support will often ask for proof of purchase from an authorized retailer, so dig that receipt out first.