You have to get injured in order to forget how to move your body parts. Your brain is what tells you how to do things, and a certain part could be damaged in order for you to forget how to do the simple task of moving your arms. Think of your brain like a computer. A computer is made up of many different functions, and if one of those functions shut down, your computer will not operate like it used to. Hemineglect, also known as unilateral neglect, hemispatial neglect or spatial neglect, is a common and disabling condition following brain damage in which patients fail to be aware of items to one side of space. This is caused by disruption in the visual cortex. So, yes, yes you can forget how to move a body part, but brain damage of some sort will definitely be the cause.