Particles disappear if I modify any parameters I seem to have a problem with particles in cycles. I'm using 2.79. I can create a particle system and use an object as the particle, but whenever I change any parameter (number of particles, rotation, etc.) they disappear not only from viewport, but also from the render. Also, when I enable the Rotation box and tweak the orientation trying to randomize it, it doesn't work, and after a couple of infructuous modification, particles disappear too. I need to know if this might be a bug or user error. 

I must say particles are being used for a still image, so I am using the first frame both as the start and end of the particle emission. I have done this in the past with success, but now it won't work. 

Thank you in advance. 

Thank you very much. Problem is solved, I was not working at the first frame, but further into the animation. I went back to the first frame and everything works as expected.
Every time you change parameters for particles, the cache for the simulation needs to be updated (re-cached).

For Blender 2.79

Press Alt+A to run the simulation again.

In Blender 2.8

Move to the first frame of the timeline (or the first frame of the simulation) and press the space bar