Theft of MacBook running El Capitan: Security Implications
Your keychain is encrypted with your login password, so the passwords stored in it should be pretty safe as long as they aren't somewhere else, too. Unfortunately, information sometimes leaks, so there may be notes, password reset emails, etc lying around unencrypted and potentially readable by whoever stole the laptop (or their fence, or...). So in theory your passwords are safe; in practice, you should reset them all.