We need more information. Start with version of InDesign, what platform you're running. Does this only happen with one file or with multiple files? What export setting did you use? What PDF reader are you using to view the exported PDF?
Well, miraculously it started working, but in case it happens again.. here is more information. I am using CS5. I have made a portfolio of my design work and only some of the files are blank. I used PDF interactive. Some of the images in my portfolio are line drawings and others are charcoal drawings. Only Some of the line drawings are the ones that are just blank when I export it.
Are they large images? Are you using Acrobat or Adobe Reader to view, or soome other application?