Our proprioception is tested by the DWI test for which we must touch our noses with our eyes closed. People with normal proprioception may make an error of no more than 2 cm. People without proprioception have no clue as to where their hands are, nor their nose.
Our muscles are like solenoids. Like solenoids, there is no way of knowing how far the muscle is streched and an extra detection system is required for that.
Without proprioception, an artist would not be able to brush paint onto a canvas without looking at his hand's action. It would be impossible to drive an automobile, because a motorist would not be able to steer or use the foot pedals while looking at the road ahead. We could not use touch typing or perform ballet. One would not even be able to walk without literally "watching where you put your feet".
Oliver Sacks reports of a young woman who lost her proprioception due to a viral infection of her spinal cord. At first she was not able to move properly at all. Later she relearned by using her sight (watching her feet) and vestibulum only. She acquired a stiff and slow movement.
Someone reports having lost the proprioception in one of his little fingers. When he grabs something, that little finger may or may not be in the way. The finger is not paralysed, it moves when he wants it to, but how far it got, he can only know by looking at it. Watching it all the time slows his work down too much, so he prefers to correct for it when he feels that little finger bump into an object he is grasping.
Apparently, loss of proprioception may happen temporarily during growth, mostly in girls of about 11 years of age. Possible experiences are that suddenly your feet or legs are missing from your mental image of yourself, or you have to look down at your legs to convince yourself that they are still cylinders that reach onto the ground, or you may fall down while walking, when you look at something other than your feet, e.g. at someone who started talking to you.
People that have a limb amputated, may still have a working propioception for that limb: this is termed a phantom limb. One person reported that when he awakes, his phantom leg is gone. He hits the remaining piece of his leg to wake his phantom leg and then puts on his leg prosthesis. Without the phantom leg he is incapable of using the leg prosthesis.