The second that you say the word "format" or "repartition", the only hope is to yank the drive from service and scan the whole surface for any recoverable private keys. Data likely will no longer be where it should be, recovery software will find junk in the sectors where "wallet.dat" is indicated to have been.

After IMMEDIATELY discontinuing use of any drive with possibly-recoverable bitcoins on it, you need to boot another operating system off another drive, and use software that will mount and scan the entire original drive by raw disk sector to discover any recoverable private key data that could be left on the drive.

Get a second hard drive to run off of or bootable USB with a linux distro on it, and get pywallet running on it: https://bitcointalk.org/index.php?topic=34028.0

Then use the command below, pointed to the appropriate device corresponding to the old hard drive and the correct drive size/area to scan, pywallet will go through these steps: 1. read every byte on the disk to look for keys 2. create a recovery file of private keys which enables 3. attempt recovery of password-encrypted keys using possible phrases

python pywallet.py --recover --recov_device /dev/sdb --recov_size 120GB --recov_outputdir ~/foundkeys

It takes about 10 hours to fully 0-wipe a 2TB hard drive, expect longer for key recovery just to read and process the whole drive.

Several threads are around discussing this, but there is not one that is at "master tutorial" level.