In your file, [*Clamp direct*  and *Clamp indirect*][1] have both been set to `.03` in *Render settings > Sampling*.

This is forcing all the samples to return a value less than or equal to .03, which results in the apparent lack of light.

Set them both to zero to disable clamping:

![enter image description here][2]

Then it should work as expected:

![enter image description here][3]


  [1]: http://www.blender.org/manual/render/cycles/settings/integrator.html#tricks
  [2]: https://i.sstatic.net/baz8f.png
  [3]: https://i.sstatic.net/iW3kR.png