If you have created a segment that has 0 subscribers in, this means that no record from the main list matches the rules you have applied in the segmentation filter.
In that case you can revisit the rules you have applied and change them in order to capture the records in the list that match the rules values.
An example of an empty segment could be that you want to see engagement numbers but there have not been campaigns in that period of time:
If you have set up a rule based on the last open date:
If there have been no campaigns sent to that list in the last month the segment will generate 0 subscribers as there is no one that matches the rule applied.
Another possible reason for 0 Segment subscribers could be if the field you have set the rules from does not contain any information (values) in the main list.