From looking at the comments, I think I can help you solve your issue.
- Get a live session running from a USB or CD.
- Open GParted while in the live session ( the drive "/" shouldn't be mounted ).
Delete all the partitions on your drive - Do not format them, just delete them from the registry.
Your data drive was lost and overwritten by your new installation, and for testdisk to recover your data drive, the new, overriding partitions need to be gone. Back up anything that you have on there and delete all the partitions.
Now because Xubuntu was installed over your data partition (assuming the data partition took up the entire space), there is mostlikely going to be data missing. Hopefully testdisk can recover what you need.
Now run testdisk and recover your data using testdisk. Look for NTFS partition, or whatever type your data partition was earlier. Testdisk WILL find your recently deleted ext4 partition, but you do not want that. You may need to do deep scans to get the data partition back.
If you need more help with this, comment here to let me know.
I have done this before (I dd'd the first hundred or so megabytes of my drive, lost everything, used testdisk to bring it all back), so I've got experience here!