As per default moodle settings, the deleted activity should go in the Moodle's Recycle been and can be restored from there within 7 days.
But sometimes it does not move to the recycle bin and also shows the kind of message "Deletion activity in progress. Some grades may be lost". In such a case we have to push the deleted activity to the Recycle bin so that it can be restored from there.
Sharing my experience. Hope it helps you too.
[DISCLAIMER: TRY THIS IF YOU ARE EXPERT ENOUGH TO UNDERSTAND THE MOODLE DATABASE AND YOU ARE ON DEVELOPMENT SERVER. ]
