Check your domain age. Lets say it is 2 months. Then you are in the Sandbox. Now lets say your site was indexed for your key phrase for 9 months. Now you are out of the Sandbox. To summarize, the Sandbox effect lasts from the moment you get indexed for a particular keyword until 2-6 months have past. The more competing pages there are for a keyword, the longer it takes. Why is there is a Sandbox you ask? So Google can prevent spam. This technique of theirs works surprisely well to weed out the bad sites in their index.
Just search for your website using any terms that you previously ranked if your website does not appear means that your website has been sandboxed.