Where are these media files?
It could be that you linked to them while they were still in the camcorder -- or to a memory card in a media slot or to an external drive -- which you've since disconnected from your computer.
You need to have the media files continually accessible to the program as you edit. You can't just connect the camcorder or a hard drive, for instance, add links to your project and then disconnect the device from your computer.
Yes, changing folder names or moving your files will also break the links. You need to relink your media files to your project using the prompt that pops up when you open your project.
Thanks for the help everyone. I was able to relink the files. I did run into a little problem . In messing around I somehow duplicated a file several times in some of the clips. I'm trying to work that out. It did teach me a lesson - get your files organized before you use them. This was a little practice project I started with when using the trial and decided it was worth keeping when I was done.
I utilize a "standard" regarding my Assets, and my folder construction. It is outlined in this article: http://forums.adobe.com/message/4028199#4028199
It does give up a bit of performance, i.e. splitting Assets over several HDD's, but makes up for that little performance hit, with ease in clean-up.
Good luck, and glad that things are progressing for you,