My Facebook id is hacked, how can I get back my account?

By 'hacked', I am assuming someone else has changed the password of your account, as you left it open somewhere and let out the password unknowingly? One way to get it back is through your primary email registered with your Facebook account. On a successful password reset, you get a password reset mail with a link to revert it back in case you didn't change the password. There are other ways to gain access if you have lost control of your email account as well. One way is to select three trustworthy friends from your friend list, who would be sent secret codes. On receiving those codes, you can use those to gain access to your account. If everything else fails, Facebook has a feature where you need to send a scan of a valid identity card of yours. The steps are explained properly there in case you have to go through it. Hope this helps.