I have a video in which the sides of every clip are completely cropped out. I have tried to find a margins preset or something, but no such luck. I have all these landscape clips and pictures, but it's showing only the center slice of them, as though they were portrait. When you click on the clip in the preview window, it clearly outlines the margins of the full landscape clip, but the side bits are totally blacked out and you can't see them. I've tried re-importing all the clips, and still no luck. If this makes sense to anyone please explain it to me. Thanks.
What is the data for frame-size of the clips as shown in the bin in the Project panel?
What is the frame-size for the sequence shown in the Seqeunce/Sequence settings dialog?
Was the first thing you put on that timeline a portrait still or clip?
Your sequence settings are not matching the aspect ratio of the original media. Choose a different sequence preset, or drag the media to the New Item icon in the Project panel. You should be set then.