Physical light sources not working

I'm trying to add lighting to my retail store project. When I started, I added some physical spot light and they worked well. But later on, when I try to add area lights, they emit no light. I tried to change the temperature, intensity, exposure etc but nothing happened. I even copied and pasted the spot light that I first added but the pasted one also doesn't emit light. Could anyone tell me what the problem is and how I can fix it? 

Below is a screenshot of my scene. The emitting light sources are what I added first. None of the other sources are emitting light.