Accounts are often hacked, including Facebook accounts. Too many times people fall for scam emails telling them to (urgently) click on a link and type in their password. Too many times people don’t know how to tell the difference between the real Facebook login page and one made by a scammer (read here for some hints).
And when an account does become compromised and hacked, the scammers usually use it to send out spam. Then it can be difficult for people like you to get that account back.
Facebook has given this problem some thought and added a way to recover a compromised account. They will send you an email and ask you to verify your account. Then on their web site they’ll ask you some security questions and ask you to change your password.
There’s more info here.