Check your friend's privacy settings. Maybe they have changed it so that only certain people can see their story. Also, make sure your Snapchat is working properly. Update it if there's an available update. And check your network connection as a bad connection can also cause this issue.
It might be that your friend has set their story privacy to a custom list that doesn't include you. Or perhaps there's a glitch in the system. First, make sure you're logged into the correct account. Then, restart your device. This can often fix minor issues that are preventing you from seeing the story.