Closing a gallery is to manually make the photos inside that project inaccessible to your clients. While expiry date automatically closes a gallery on the date set by you.

Reopening a gallery you had closed is easy!

Choose any gallery you wish to close manually. Navigate to the left side column and find Gallery Settings. The last option reads Reopen Gallery which will immediately make the gallery available for client. Note that this is the same procedure to follow while closing a gallery.