If they reformatted the hard disk and then re-installed Linux, and the data was kept on the disk that was reformatted, then the data is permanently lost.

If this is not the case, you should be able to find the data on the hard drive with Linux if it is still there. Try a search. Maybe it has been moved to a different location on the system.

Also make sure the data was not kept on an external hard drive, which would need to be plugged in for it to show up on the computer, obviously.