Inactive segments are hidden in the default view of the Segments list, My active segments. To see them, change the list view to My inactive segments or All inactive segments.
You can't send an email campaign to an inactive segment. The New email campaign button on the segment is disabled.
When you make a segment inactive, any flow using it is paused, and no emails are sent as part of the flow. To start the flow again, make the segment active and then click Activate flow.