I have migrated from Windows 7 to Ubuntu just yesterday. Windows 7 was in my C: drive. I did not realise that if I wipe Windows 7 and install Ubuntu, Ubuntu would be installed in the C: drive only. So I didn't make any backups, and I deleted Windows and installed Ubuntu. But, Ubuntu has formatted my hard drive and I have lost all my important data!
How can I recover my data now?
For recovering accidentally overwritten partitions with TestDisk 
please follow these prcoedures:
Optionally (but highly advisable) and if we had an external drive big enough to hold all of the faulty drive we can make a backup to recover data or partitions from an image. Make this image with the following command in a terminal:
sudo dd if=/dev/sdX of=/path/to/external_drive/backup.img
Replace sdX with the drive to recover. Since dd is an advanced and potentially harmful tool take extra care to not accidentally overwrite any drive you may still need.
In case you are unable to recover your partitions you may be able to recover single files by running PhotoRec wich comes with the TestDisk suite.
Just to have mentioned it: there are other "forensic" tools to help you recover lost partitions or files. See How to recover deleted files?.
(For those who have the opposite problem: you think you've formatted the Ubuntu partition).
1) BAD SCENARIO It either should display that the partition is still using Ubuntu filesystem (typically ext4) or not. If you've formatted the Ubuntu partition (ext4) in Windows to the NTFS filesystem, than the data are very likely gone. :-(((
2) Good scenario If the partition is there and you simply cannot boot to the Ubuntu, try this guide to restore the (grub) boot chain:
You can use the testdisk application to recover non-overwrriten data. Use an Ubuntu Live CD and install testdisk on it with the following command:
sudo apt-get install testdisk
Keep in mind that you should not use the Ubuntu partition to install testdisk on!
