my MacBook pro got locked after I signed in to iCloud

Hello,

I have a used MacBook pro laptop that I purchased from my co-worker two years ago. I've been forced to change its motherboard after a water damage.

Yesterday I thought about giving iCloud a try to backup my data. Once I signed in using my account I got a message with the title "Find My Iphone" says it found the Macbook then the mac restarted showing a blinking folder with question mark icon. I tried turning the mac ON with pressing the option button and I got the passcode icon.

I tried to reach my co-worker and he told me he does not remember the lock code and he removed the device from his icloud account a long time ago.

What should I do?