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Recently while trying to render fire I run into a problem where the fire shows up in preview but it does not render, only the light that comes from it. 
All the settings are at default, I can't seem to find what is causing this as I was perfectly able to render fire before. 
I recently had a similiar problem. Your render, in fact, is correct, but the result is displayed incorrectly. This is due to the way alpha is represented in the viewer, which uses straight alpha rather than premultiplied, so emissive volumes like fire can't be properly modelled. For a detailed explanation, see this answer.
To see the fire, just try switching to RGB mode instead of RGBA in the image viewer and you'll see the flames:
As suggested in the linked answer, a solution would be to use an EXR image wich has premultiplied alpha. If you can't use an EXR however, you need to convert the premultiplied alpha to straight alpha in the compositor, then save as e.g. PNG:
I also have this problem every now and then. For me neither solutions worked, the converter nor the EXR render.
For the people that also can't do those two things. I simply rendered out an emission pass (for animation).
In 2.8 that is easily done. 
