I have a scene where the sun rays should be reflected from a mirror to create this sort of effect:
The problem below was resolved, scroll down to EDIT.
However, I can't get any light reflected from any glossy surface. I set up a simple scene to test it ans here is my result.
And here is rendering.
But the cube stays dark.
I will be very happy if somebody could help me with this.
I fixed the problem with the test file by adjusting light position and size (and maybe something else). For that I added a ground plane and was adjusting the parameters until I got the shadows right.
Increasing the roughness parameter, suggested by Duane Dibbley makes rendering faster and less noisy. However, then it is not a mirror any more. When the roughness is zero, rendering takes a lot of time and doesn't look like it is going to finish.