How to log on to macbook pro after its stolen?

My house was burgled a few days go and my mac was stolen, fortunately the police managed to track the theives down after find my iphone gave them a lead, i managed to lock the mac via find my iphone, and got my laptop back yesterday however after entering the passcode to unlock, the actual username is of someone else with a password so cant get in, its definitely my laptop with the serial number, how can I log on? And does that mean they've wiped out all my data? Also, how was it they were able to erase and create a new account if i've locked it on find my iphone? It didnt link to the internet till 5 days after its been stolen 😟