By disappeared you mean "it's deindexed" or you just lost the first page position? If it is the latter, then it's probably a Google Dance. Give it some time.

There's so many variables to that question and its impossible to give you a proper answer without you answering a bunch more questions , and even then it will still be just an educated guess.

Check whether your site appears if you use this search operator - site:yourwebsite.com
If your site does not appear then it is deindexed and next step you have to resolve what made your site deindex and submit a request to include your site back to google.

If your site appears after using the above operator then it is google dance and just add few good blog posts and build links, it should be back in a few weeks to its position.

I just googled how to find out if a site is deindexed by google and there's not a ton of good info out there. glad royalking knows a quick easy check.