Powerpoint, can't save to documents

I am a teacher and my kids always were able to save to the documents file on the hard drive. When they do, we get a message that says, Powerpoint cannot access macintosh hd because the file has been moved or deleted.

Any suggestions?

Moved to proper forum and title edited for clarity.

Have you verified that the document you're trying to save to actually exists in the user documents folder? It may have been moved or renamed.