Unfortuantely, it would seem you are boned. From what little I understand about partitioning and the wiki article regarding ext4, the Xubuntu boot was in the process of wiping your partitions and creating its own. This does not, however, mean necessarily that the files are lost, but certainly enough information was changed or lost that the computer no longer recognizes how to boot the drive.
You can try removing the drive and, if you have a hard drive enclosure, a universal usb drive adaptor, or some other way of hooking up your PC's hard drive to a different and functioning computer. With any luck, your drive will appear just as any other external hard drive would and you may be able to access some if not most of your files. If it does not, or most of the files don't appear within the drive, there are various free and costly softwares available to attempt to recover the lost data.
This above link offers a multitude of software. Barring any of these working your only remaining option would be an even more expensive professional recovery which carries 0 guarantee of success.
Fortunately, once you get your system up and running again, steam can re-download and reinstall all of your games. Though accessing your steam account from a "new" computer may be your biggest obstacle here.
Hope this helps and best of luck.