It could be any number of issues... odds are you will need client care to see if there are more detailed logs in the server as to what failed...
Since the job is running now, I would guess that there was likely a temporary issue (either at Adobe's end, or yours that caused an issue).
To get back-fills of the jobs that failed, I believe you can go into your Data Warehouse Request Manager, and click on the name of the scheduled report to bring up the Request History for that file.
You should see a table showing each run, with an ID, Date, etc.. on the right, there should be a "Resend" option. When you click on it, it should ask you if you are sure you want to resend the request (quoting the request id).
This should result in the file that will match what was intended to send on that date/time.
