maybe you were very, very bad for google . You can show us the URL of your site to see what may be the problem.
Disappeared as in?? Did it get penalised and dropped in rankings or vanished completely? If latter, then I would suggest you to wait a little. This just might be Google dance and your site could be back in rankings in a few days.
Cloaking is the most common reason why people are banned. Sometimes you don't even understand that you are doing it. But google's own judgment rules.
Maybe you got "Sandboxed". It usually happens when you spam your links everywhere. One method to get out of a sandbox is to build links as fast as you were before but gradually slow down... or gradually go up when you are building. Or its just Google playing something on you, except to be back in a few days... if not, complain in Google Webmasters Central.
This is because some recent changes in the Google algorithm, have a patience and remove the spam of your site..