Can I edit a movie after I have exported?

I made a movie and exported it to my computer without finalizing. When I went to watch the movie back I noticed a mistake. I still have the project in iMovie and all the files and all I need to do is crop a section of the clip off but I can't click on the crop selection, A message came up saying that I had to do something before I could edit again but I missed it. Can anyone help me?