The 'unity 3d light not working' issue can be attributed to several causes:
Incorrect Light Settings: The settings for your lights might not be properly adjusted, which could keep them from illuminating your scene as expected.
Light Baking Problems: If you're using baked lighting, the problem could be due to incorrect baking settings or a failure in the baking process.
Shader Issues: Certain shaders do not interact with lighting as we might expect, and if your material uses one of these shaders, it won't respond to lighting.
Camera Clipping: In some cases, the camera's Near and Far clipping planes might not include the lit objects, causing the light not to render correctly.
Solution - Here's How To Resolve It
To resolve this issue, follow these steps:
Check the Light Settings: Make sure that your light is set up correctly. Check its intensity, range, and color. Also, ensure that the light is actually targeted at the object you want to illuminate.
Re-Bake Your Lighting: If you're using baked lights, it might help to clear the baked data and re-bake the lights. You might also need to adjust your bake settings.
Check Your Shaders: Examine the shader used by the material on the object you're trying to illuminate. If the shader does not support lighting (like Unlit shaders), consider switching to a different one.
Adjust Camera Clipping Planes: Adjust the Near and Far clipping planes of your camera to make sure they include all the objects you want to be visible and affected by light.