If you formatted the drive you need a Forensic Recovery program and you shouldn't be using something like Stella for this type of Recovery. All you'll get is corrupted Data well most likely File Headers but not the complete file. ;)
If you just installed over an existing OS without Formatting something like Stellar will work but you'll need to be sure that the client is willing to pay for this as it's both time consuming and not straight forward.
Then supposing that the drive wasn't previously Encrypted or the files where not encrypted with some type of Encryption Utility it's just a long time to do which you should be paid for and here I would fit a new HDD to that system to prevent any more Data Loss. Then work with the old HDD and [b]Never[/b] write anything to it till you are certain that all the data is recovered. Then fit it to an External USB Case and return it to the client as a Backup Drive.
With Formatted HDD your best bet is probably something like On Tracks Offering that can rebuild Partition tables but this isn't cheap it's over 1K and I think it's worth every cent and then a lot more but if the client isn't willing to pay and you don't have a ongoing need for this type of software it's way too expensive for a One Off use.
[i]edited to add the forgotten link[/i] http://www.ontrackdatarecovery.com/file-recovery-software/
Col