My backup files are missing

I saved all my backup files on my passport from Vista and then installed XP. When I reinstalled the WD Sync program on my comp and plugged the external hardrive in, my data is nonexistant. What is going on? Was my data written over because the drive simply synced with the data currently on my comp? If this is so then this is horrible backup software and I won’t be using it again.  I’ve never been so angry. Thank you.