Constituent based query lists pull directly from the Raiser's Edge.  If the query in Raiser's Edge is no longer returning any results, the list in NetCommunity will not be able to send to recipients.  

Log into the Raiser's Edge, and run the query again to make sure it is still returning results.  If not, adjust the criteria as required.
If the query is still returning results:

1.  Check that all of the recipients have an email address on their record, and ensure that the NetCommunity phone type mapping is not pulling from the wrong Raiser's Edge field.
2.  Make sure that there are no organization records in the query.  NetCommunity currently can only email to constituent records.
3.  Log in to the NetCommunity site, and edit the list under Emails, Lists.  Click the button to refresh the list, and monitor for errors in list refresh.

Note: If none of these steps are able to resolve the issue, this may indicate that the list has been corrupted.  Please try creating a new list, and uploading the constituent based query again.