Since I wasn't able to follow the other answer here I figured out an alternative and thought I'd post it in case it would help anyone else.
What I did was go under Stories, tap the symbol in the top right corner, and type in the name of the story I hid.* It then popped up and I was able to tap the symbol to show the story again.**
*The story I accidentally hid wasn't something I followed (it's automatically provided by Snapchat), so I couldn't go under My Friends to change anything.
**When I go into My Friends I don't have the symbol Pete mentioned. That symbol only shows for me if I go into the Stories section.
Hope that helped!