I'm bumping this rather than starting a new topic because Polyglyphs described the problem so well.
When I discover that the render is hanging, I try pausing it, at which the screen suddenly displays more of the render, as if it were doing the job but not deigning to show me. Then I hit resume, and listen for the CPU fan to know whether it's continuing or not, because the panel no longer shows me the elapsing time. Sometimes it takes a number of "prods" to complete a render.
When it shows "Preparing to render..." but doesn't actually start, I close the program and restart, and that usually helps. I was experiencing just such an endless "Preparing" when I opened this forum, searched for history on the problem, and started this post. I was writing this paragraph when the CPU fan suddenly whirred to life, and lo, now I have a completed render.
Sometimes the RTP button does this, too. The preview prepares forever, or simply doesn't happen; it might say "finished", but the preview is still showing box bounds, not objects. My scene is in its early stages, with no vegetation, no populators, just water, terrain, shaders, and a handful of objects.
Does Terragen avail itself of the GPU memory or what I think you call CUDA? The software for my NVidia board (GTX960M, 2GB of RAM) lets me direct certain apps to use it rather than the integrated Intel graphics, and I did so with Terragen, but I can't tell whether Terragen is using it.