iCloud hacked?

Hello,

So I switched on my MacBook the other week, logged in and then opened Safari. The next thing I know, my MacBook restarts itself and I’m met with a grey passcode screen.

Turns out my MacBook was locked via iCloud. Only my iCloud acc. is linked to this MacBook, and I don’t know what the passcode is - and how it was locked in the first place!

Any ideas as to what may have happened?

After many hours of research, it turns out that only my iCloud could have locked it. Also, you set the [6-digit] passcode as and when you lock it, and so I have no idea what it is.