CSV Import Undo issue

During the course of using the CSV Import plugin to migrate an Omeka.net site into a local installation, I found an error in the CSV file that made it "skip" all the items in the import. On the "Status" tab of the CSV Import page, I meant to click the "Undo Import" button for the collection that I had just imported incorrectly, but I clicked the button for the wrong collection and accidentally reversed the import of a huge collection that was ingested a year ago from a different Omeka.net site that we no longer have the CSV file for (and the site is no longer around to get a new one). Now all the items and metadata from that collection are gone, and there is no option to redo the the accidental undo that I did. The only option available now for that collection is to "Clear History".

Is there any way to get these items back, or are they gone forever?

Also forgot to mention, I am using Omeka v1.5.2 and CSV Import plugin v1.3.3. Thanks in advance for any replies.