On the new version of Facebook, there is a side bar down the left hand side of the screen. At the bottom of this side bar, there is a little cog shaped icon. Click on this, and it will bring up your options for chat. If "available to chat" is not ticked, click on it to "tick it", and then you should be able to chat.
You can also enable or disable chat sounds by ticking it or clicking again to disable it here too.
Enable chat on older versions of Facebook
If you choose not to update to the latest version of Facebook, then the option to be online or offline should be available on the right hand side of your screen where you click to get a list of any friends that are online. Here, the options are online or offline rather than "available to chat". If it says that you are offline, simply click on "online" and this should make you visible to all your friends online.
Problems with Facebook chat
Sometimes, Facebook has technical issues due to too many people being online or maintenance work going on on the site. This could result in Facebook chat being unavailable until the problem is resolved. This could be why your chat is saying "disabled" rather than "offline". These issues usually only last a few hours so try refreshing your page or logging in and out again to get your chat facility back.
This can also happen during the period in which Facebook updates itself to a newer system. Once you have the latest version, you should be ok. Again though, if it still won't work, log out and in again and refresh the page to try and resolve the issue.