If there was a whole civilization of people without sight, how would they imagine things? Would they be able to picture 3 dimensions in their heads by feeling things?
Of course. After all, blind people in real life have been composers, painters, and authors. Most things in life have multiple sensory aspects. A blind person could sculpt by feeling the geometry of their medium (for example, Felice Tagliaferri). A blind artist could paint by knowing the relation of one line to another, which is likely proprioception-based. Such an artist could also lay down a textured surface as a guideline. One example would be John Bramblitt (https://en.wikipedia.org/wiki/John_Bramblitt). Composing music, an imaginative art, is of course scarcely more difficult for the blind individual than for the sighted(compared to the previous arts). Perhaps a few people here have heard of a guy called Stevie Wonder? (https://en.wikipedia.org/wiki/Stevie_Wonder). A blind person can even fly a plane (http://planelopnik.kinja.com/blind-pilot-flies-and-lands-a-real-airplane-1657930420). Talk about destroying stereotypes. I imagine that for someone who can conceive of the position of the plane relative to the ground, flying blind presents relatively few obstacles (though keeping it level might be hard).
All of the previous examples were merely for modern human societies. Since this question is in Worldbuilding, perhaps non-human senses or advanced technology is permitted. In that case, there are so many more possibilities for sensing 3D geometry: echolocation, thermoception, telepathy, magnetic fields, air currents, even whiskers!
This question is asked by someone who uses their sight more than their other senses. It is proven that when you loose your sight the other senses compensate and are more significant in providing feedback.
Spacial sense can best be imitated for one with sight by turning all lights out and navigating spaces you are familiar with. While you may bump into things, that all changes as you get more familiar with the steps required and placement of those objects. You have a visual memory of that space, but if you do that with a room where someone moves things around and you then navigate that space in pitch black, you will eventually learn what that space "looks" like.
Music and sound are important components without sight. There are blind people who do a form of bat echo-location by listening to the space they are navigating. They will recognize people around them by their breathing, their footsteps, their voices
Touch can also indicate different environment conditions. The air moving around your body can tell how open a space is. The feel of surfaces under your feet can indicate where in a familiar space you are in. There is a tongue device that is similar to the toy where people press their faces or hand in from one side and the rods extend forming that shape on the other... in this instance the rods form a "picture" of the physical space that is read by the wearer's tongue and helps them "see" through touch the space around them.
Smell is another sense that adapts and becomes more pronounced. Smell like touch can give indicators to space, where someone is, who is in that space with them, what objects are present.
Art and imagination in this kind of world will take on things that stimulate the senses. Sound is an obvious choice, but tactile, food, and aromatic forms of art also would be highly appreciated.
The tools and techniques that would develop in a large group without sight might surprise people who depend on sight for most of how they interact with the world.