My guess is that you deleted the topic using Windows Explorer? If that's how you deleted the topic, you can't work that way. You need to delete the topic from the Project Manager pod so RoboHelp may update all references accordingly.
Helpful and Handy Links
To get rid of the broken reference now, just put a file with the same name in the correct spot (can be a blank text file as long as the file name and extension match). This will get rid of the red x and you can delete from RH normally. (additional hoops if you use source control, though...)