Depending on your theme there may not be any code for a horizontal rule in the stylesheet, I don't think the popular Hiero theme has one, but don't despair, there are two ways you can insert a horizontal rule:

You can create your own css {hr} code or install the Tinymce Advanced plugin. Once you install, update and activate the plugin go to your text editor and click on the visual button and you should see something like this:

Click on the horizontal rule symbol, ringed in red, and you should see a horozontal rule like the one below: Unless your stylesheet has a hr code, which will show the horizontal rule according to the code, by default the horizontal rule will be black and the width of your body text.

You can change this in you stylesheet if you have one, if you haven't don't worry, we'll come to that. Below is an example of changing the hr code:

Which do you prefer?