As per the FAQ How do I force the reports to be re-processed from the logs? there are several methods.
You can drop the tables piwik_archive_blob_YYYY_MM
and piwik_archive_numeric_YYYY_MM
containing the date you want to be rearchived. This is the method I had used.
Alternatively you can temporarily add:
[Debug]
always_archive_data_day = 1
to your config/config.ini.php
to force archiving of daily reports.