Is LED or flat broken?

Hi, my macbook started to become dimmer with the passage of time and now the left lamp remains off while the left is functional but not at the level of factory brightness. Sometimes, just lit both lamps operate at an acceptable brightness after a few minutes left lamp becomes bright as soon as I bought the computer and after a few minutes turns off. I tried to reset the SMC without success!

It may depend only on the cable or the screen is broken?

Thanks in advance