Okay, so first, I would check if you have a directional light source and point it towards your scene. If you do and there's still issues, then do this...
- Create a new material, with shader Sprites > Diffuse.
- Select all the objects you want to react to lighting, go to their Sprite Renderer and change the material to the one you just created.
- Create a point/spot light at Z=-1 and you should see the light.
This is mainly for doing small space lighting (i.e. dark room with candle)