Closed wikis are wikis that existed but were recently closed. Such wikis might still have their database dumps available for download provided they weren't closed for content policy violations. Users may also request that Fandom consider reopening a closed wiki by submitting a support ticket. The request will be reviewed by Fandom and, if they agree, the wiki will be reopened.
Invalid wikis, on the other hand, are wikis that either never existed or that used to exist but got closed and are now past the retention date. Wikis that got closed and are now past the retention date generally cannot be reopened and the database dumps are likely no longer available. My understanding (which may not be correct), is that wikis that remain closed for 30 days are then made invalid wikis.
