I find this online resource handy:
http://project5.technetos.com/faq/ Hopefully this will help. usually if you haven't opened a project for sometime there might be a missing plugin (this includes all other installations of Cakewalk Software as all the plugins would be available at the time you created your project) or the audio folder has been moved so you might check where the project was saved to see if the audio folder is still intact, best of all luck my friend.