Without more details (like, is your project made up of different video sources) it's hard to make many suggestions.
One thing you can do is check your project's resolution and aspect ratio.
On a good segment, right-click the video track and click "Object properties". You will see resolution (i.e., 720 x 480 pixel) and Apsect ratio (i.e., 4:3).
Make sure your problem segment is set correctly. The resolution can't be changed, that's dictated by the original video, but you can change the Aspect ratio, which might be your problem.