I deleted saved picture album by mistake from my Lumia 820. How can I return it back? I didn't activate the OneDrive service before deleting the album.
The typical back up options are either through OneDrive or by syncing your phone with your PC. In the absence of these, those photos are gone forever.
BTW, in case you are wondering about whether you can use file recovery software to get the photos back: unfortunately, not.
When a smartphone is connected to Windows, the device is not treated as a regular external disk and so it doesn't get a drive letter assigned to it. Since it doesn't have a drive allocation, file recovery software isn't able to detect it. If you are looking for more technical details, read on here.
There is no undelete option, and the recommended route is to back the photos up yourself, either by using the OneDrive integration, or by regularly syncing with your PC. Personally, I take both approaches, on the basis that phones (whatever their make/operating system) are incredibly easy to lose/damage beyond use