Welcome to the forum.
I suspect you renamed the source file using Windows Explorer
rather than doing it the correct way using File | Rename Project.
If that is the case, then the missing files will have names that no
longer match the project file and RoboHelp has lost track of them
as a result.
What you need to do is put the project file name back to what
it was. Then open RoboHelp and you should find you have everything
back. Assuming that is the case, use File | Rename Project and you
should be OK.
Help others by clicking Correct Answer if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.