In all likelihood, you're pretty much screwed. It wouldn't hurt to try a raw un-delete on the off chance that every last byte of your stuff was not over-written. A number of un-delete programs have a low-level or raw mode that scavenges the hard disk for snippets of anything that is not a hard format pattern and links together what it can. The results will potentially contain whole files, partial files, and cross-linked files (pieces of old files grafted to pieces of new files because a link in the chain was over-written). If it does find whole, uncorrupted files, that's when your fun will begin, as you try to figure out whether any of it is your files and if so, which ones.
Do the recovery as soon as possible because anything written to the hard disk could over-write your old files if the data is still there. Do the recovery to a different physical storage medium so that the recovered files don't, themselves, over-write other recoverable data.