You haven’t given enough info to answer definitively, only to guess. Please take the following as editorial guidance for all people posting requests for help, not as a personal attack:
- the phrase “after creating a face and modifying it” is very non-specific. There must be millions of ways one could do that. What did you actually do? How did you create a face? How did you modify it? What tools or extensions did you use?
- you say “there seem to be breaks in the faces” but don’t point any of them out. What specific problem are you asking for help with?
- your picture appears to be in X-ray mode, which doesn’t help to illustrate your point, as one can see through both valid faces and gaps.
- your picture doesn’t provide a lot of clarity or context as to what is really going on. Posting a model would be much better.
- you refer to “the little black line”, by which you presumably mean the tiny edge at the center of your picture, but “a segment disappeared”…what is a “segment”? Do you mean an edge? A face? Which “segment” disappeared?
With all that said, here are some off-the-cuff suggestions based on typical issues:
- Inability to create a face is almost always due to the corner points not being co-planar. This can usually be detected by drawing a diagonal edge. If a triangular face suddenly appears, the other corners are not co-planar with the three you chose.
- If you created the drawing using Follow-me or Intersect with, have you tried scaling up first to avoid issues with very small geometry?
- Have you tried View->hidden geometry to see where there are hidden edges that might be affecting what you are trying to do?