Diagnosis
The infrared LEDs for night vision are either dirty or there's an object blocking them. A layer of dust or a stray crib bumper can completely scatter the IR light, ruining the image.
How to Fix It
First, power off the camera. Look at the ring around the lens—that's where the IR LEDs are. Use a dry, soft microfiber cloth and gently wipe that entire ring. Make sure there's nothing physically in front of the camera, like a mobile, stuffed animal, or the edge of a crib bumper.
If That Didn't Work
The camera might be in a very dark room. The night vision needs a tiny bit of ambient light to perform best. Try adding a very dim nightlight in the room, not pointed at the camera. Sometimes that provides just enough contrast for the sensor.
Heads Up
Don't use any cleaning fluids or water on the lens or LED ring. You can permanently haze the plastic. Dry cloth only.