An album that's marked as deleted will be purged permanently after 72 hours. Within this timeframe it is still possible to recover it as follows:
Start by clicking on Albums in the top banner. This will take you to the Albums page.
You will not see deleted albums by default, so you will need to click on the trashcan filter icon at the top right to display it.
Then click on the album to open it.
Now, click on the trashcan icon in the left side panel to toggle the deleted setting. This will make the archive active again so you can keep on working with it.
Tip: If you simply want to "close" an album after a project has been completed, for instance, you can simply archive it. That way, it will still be accessible, but users will not be able to add or remove content from it.