An annoying issue in Word 2013 is that sometimes the space key seems to stop working, until you press a non-space character, at which point Word deigns to show you both the space and non-space characters.
To solve the problem, press enter to create a new line, then go back to your line. The problem is caused by a bug in Word where having a page break or section break right after the line you are on prevents spaces from showing. Make sure there is a line (empty or not) below the line you are typing on, and the problem disappears.