A podcast series gets archived if the feed has been inaccessible for some time, preventing our system from retrieving updates from it. It's also possible that the series has a new feed now.
You can verify this by contacting the publisher directly, and comparing the feed they may provide to the feed we have in our catalog.
To check the series feed, simply access its detail screen, tap on the 3-dot menu next to the search icon, then select [Links...] > [Series RSS feed]. If the feeds are different from each other, please feel free to reach us at firstname.lastname@example.org so we can have it updated for you.