Since your gallery list displays only your active (non-expired) galleries by default, the first step to reactivating a gallery is to display your expired galleries by doing the following:
Head to the main menu and click Photos » Galleries.
Once you're viewing the list of galleries, click the drop down box labeled Quick Filter.
Choose the Expired Galleries option.
Once your expired galleries are being displayed, you can reactivate your gallery as follows:
Click on the name of your gallery and scroll down to the Access section.
Set the Status to Allow Visitor Access.
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. The maximum is 9999 days.
Click Save.
To Enable Multiple Expired Galleries at One Time
This option is helpful when you want to run a sale on previously expired galleries.
Head to the Photos » Galleries page
In the upper right under Quick Filter: choose Expired Galleries
Also in the upper right corner make sure you have chosen the List View option (three lines).
Put a check mark next to any gallery that you want to quickly reactivate.
Click the Actions box in the upper right
Click Change Expiration Date.
Select the date that each of these galleries would expire.
Click Save.
Each of those galleries will automatically be activated and will also expire automatically on the date you selected.