Could a planet without a Sun sustain human life by internal heat? If you have a planet without a Sun that has enough internal heat and has a thick upper atmosphere that traps most of the electromagnetic radiation I think that it would be warm enough to have water and it should be possible to sustain human life by internal heating. Is this theoretically plausible or am I missing something?
Could enough heat be trapped so that water is fluid but the surface is still a solid cool crust as we know it?
Even if only a bit of visible light is emitted from the planet (because it probably will have a lower temperature black body spectrum than the Sun), if this visible light is trapped by the atmosphere, could this accumulate to a large amount of visible light as we know it on Earth?
 A: No as we would also need sunlight for photosynthesis to create an atmosphere. We could artificially do this but the power needs would be huge and if there is no sun then that rules out solar panels. Wind turbines likely wouldn't work as they rely on wind which rely on different temperatures on earth due to the angle that the suns rays hit earth.
From the question I also gather that you are wondering about visibility. You could use lights. Your point on black body heat is true. We may have evolved to see using ir rather than visible light (which would have a different name).
I think, not sure, We best see orange because that is the most emitted light from the sun.
Also as a power source one could use geo thermal which is what a comment said.
A: We need to both comfortable temperatures and an energy source that has a far lower entropy per unit energy compared to energy from our local environment. Sunlight provides for such a low entropy energy source, this is needed to keep lifeforms (which are ultimately processes that are far from thermal equilibrium) alive. Without sunlight you can use reactive chemicals, which is good enough for primitive microbes but lethal for complex life. 
A: Human life depends on a life sustaining eco-system, and a human life sustaining eco-system depends on periodic solar energy input.
Humans can get by for a while in an artificial environment on the hypothetical planet...  but it would not be the planet that's sustaining them.
We depend on wildlife and plant life in a food chain -- both of which depend on a day/night cycle of solar radiation. Some species also depend on seasonal variation in solar radiation.  Some species at the bottom of the food chain also depend on tidal actions...  Though you didn't mention if there was a large moon or not.
We also directly depend on solar radiation for our bodies to manufacture vitamin D -- an essential vitamin.   People on the hypothetical planet could get by on supplements, but those supplements would either have to be artificially made, or be brought from a planet that has a sun.
So no, a planet without a sun can not sustain human life, in the normal sense of sustainability.
A: Human life, unlikely for the reasons stated and @scm's good answer.
Sustain life could be done with many artificial means, so the question should really be grow and maintain life.  
But if you'd ask for intelligent life instead of human life it's harder to answer. It'll need, yes energy, and some environmental diversity to evolve, as well as an entropy dump. If it did it would need time to evolve. It'll certainly evolve differently, maybe purely aquatic in a place like Europe (Jupiter's moon), maybe something terrestrial underground. But time would be a key factor: it took about 4 billion years for earth to develop humans, and not sure that a planet could sustain some reasonable environments over that long. 
If you just ask for any life, or multicellular life (or just some complexity if they are not cellular), more likely as time needed would not need to be as much. 
Anyways, the physics might provide energy and entropy disposal, and not too extreme a temperature, but astrobiologists may not better than physicists what else it takes
