My laptop on layby sold to someone else

I was paying for my laptop layby for the past three months at their centurion branch. I come to collect and they don't have it in stock. How does that happen?Someone took my laptop and sold it. How unprofessional. Not to mention how in every one of their stores they seem to have people who are clueless about their jobs and constantly asking what to do. So disappointing really. I am really done with this store for now.