Networkdays function combined with If Statement

The spread sheet I am working on is for project that are"not started yet" "in progress" and "complete".I am using the networkdays function to see how many days a project takes, from this days result i check it against todays date function and it gives me the days Completed in % . It is working fine for projects that are in progress. The issue is with project that have finished or are not started yet. I think I might need and If statement but am not sure where to start. I tried a few thing but didn't get any results. Any Help would be welcome as I am an excel beginner.