Is this your only sequence with this problem, or do you have this all the time.
Anyway, this quick tip might help.
tip 1. Create a new sequence, copy all your clips from the damaged sequence and paste it in to the new one.
tip 2. you can recall the source of a clip by positioning the playhead over the clip in the timeline en then hit the ‘f’ key. Now you can drag this clip again from the viewer and replace the damaged clip.
tip 3. Rerender, maybe the render files are damaged.