Even if the master DVD video you are trying to render from is 24FPS, you should still see something. Perhaps you dropped the opacity level accidentally, or you clicked off the line with that video segment. Run the cursor over it in the timeline and see if it shows up in the video preview.

You might also have forgotten to include it in the range of video that you wanted to render. Is the audio dropped as well? If there's a long stretch of nothing where the video should be and the audio is gone as well, then these segments may have somehow been clicked off.