My Mac broke down and I had to send it to the repair centre. Unfortunately, after it came back I reinstalled Lightroom but no sign of my previous work. All the presets I made were also gone. How is that possible?
Tell us more about what happened and what the repair center did. Tell us more about what you did once you got the computer back. The more details you can give us, the better chance we have of helping you.
... but no sign of my previous work. All the presets I made were also gone. How is that possible?
I'm not a Mac user but in this case the operating system doesn't matter. I think the repair center has reinstall you Mac and they can only install the operating system. All other applications that you have owned and all your data must be reinstalled or restored by yourself.
Do you have a backup of your catalog and your pictures? If yes you can restore your work. Otherwise I think you're unlucky.