If you are updating your database with file import and uses the replace import method you may have noticed the label inactive showing by a contact in your database.
After a replace import, only the contacts included in your file are active and can receive communication.
I.e. contacts that were in your database before the import but were not included in your replace import will become inactive after your import. To become active again, they will need to be included in a new import (with any of the methods add or replace).
Basically, what defines a contact status is:
Active: the contact has a modified date that is equal to or after last (replace) import time.
Inactive: the contact has a modified date that is older than last (replace) import time.
Inactive contacts will not be included in message generations or testing segments but will be included in export jobs if requested.
To export inactive contacts, select the strategy All contacts in list, and include DateModified. A contact with a modified date prior your last replace import is inactive (you can of course create a segment targeting only these contacts but note that testing such a segment will not target any contacts, but running the export will).