...the file I was working on...
You don't state what file type this file is so I will assume that it is a Story Project HTML file, like those found within the folder accessed via the Install-able release of the Twine 2 application's Twine > Show Library menu items.
If you open that file with Notepad the file should contain markup elements like the following, which I have reformatted to make it a little easier to read.
name="The Story's Name"
ifid="...a unique set of numbers and characters..."
<style role="stylesheet" id="twine-user-stylesheet" type="text/twine-css"></style>
<tw-passagedata pid="1" name="Name of Main Passage" tags="" position="412,186">...contents of Passage...</tw-passagedata>
<tw-passagedata pid="2" name="Name of Another Passage" tags="" position="412,220">...contents of Passage...</tw-passagedata>
The <tw-passagedata> elements represent the Passages of your story, each one must have an unique value for both the PID and NAME attributes. If there are no <tw-passagedata> elements within the file then your story contains no Passages.
The parent <tw-storydata> element represents the story project itself, and most (all?) of its attributes are mandatory. The startnode attribute indicates the starting point Passage so its value must be the PID of one of the <tw-passagedata> elements within the file, if it isn't then the story project HTML file may not import.
If the story project HTML file is missing any (all) of these elements, or if the parent-child structure of the markup elements are invalid/malformed then the file may not import.