The most common reason that this happens is because one of the two settings you need to adjust has been overlooked. To successfully reactivate an expired gallery adjust the following two settings, and then save your changes:
Set the Active Days to a number high enough to reactivate the gallery. Note: this number counts up from the date that the gallery was first activated.
Set the Status to Allow Visitor Access.
If you need additional help with reactivating an expired gallery, please see the the following article: How can I enable a previously expired gallery?