Oh man...that so blows Binxgook
Especially since I don't think there is a way to recover data from a formatted drive. Once a drive is formatted...it's a clean slate. There may be some high tech recovery company that I'm not aware of...but IF there is...it's probably gonna cost a bundle.
Really sorry for the bad news
As to why it happened, hard to say for sure. There are so many nuances to working on computers...could have been anything from accidently selecting the wrong drive for formatting...to not having the master/slave relationship correct. Or, something else entirely...
Doesn't help you now, but I'd strongly recommend buying an external usb drive for backups. Drive space is cheap these days...and it's good peace of mind to know your data is safe.
Again, really sorry man...wish I could be more help