I feel your pain, having done something similar once (ok, maybe twice) before. Unfortunately, the best you can do is the nightly backups that Mendix generates, unless you or another developer happened to make a manual backup, or downloaded a backup with some of the deleted data, at some point.
Sorry I couldn’t provide you with a better answer to this one,
When you update an app always make a backup copy. So when you do revert make a copy first of the current situation and then revert back to the previous version with that backup. Then at least you have a local copy with the latest data and can do an analyses of what is missing and how to get that data to the previous version. Not an easy task by the way unless your app is small.