Any collections deleted are "soft" deleted for 30 days. If needed you can recover a deleted collection within that 30-day grace period. The collection will return to the original position with the original access rights and sales options.
To recover a deleted collection:
Go to Control Panel -> My Images -> Bulk Delete Collections -> Recover
Locate the collection (highlighted in red) and tick the checkbox to the right.
Click "Recover Collections"
(Note that collections highlighted in brown are those scheduled to be deleted in the future using Timers.)