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i found this from one site SEOMOZ. I hope this helps everyone that is in confusion of what has happened on their site. is it banned? sandboxed? etc..
and here are my simple thoughts to check if your site is banned:
1. Type my domain on google search. if it appears then its not banned.
2. If for a specific keyword your site did not appear, after weeks or months that it appeared previously, then your site is keyword banned.
3. If by means, you dont appear on all of your KW targets, then maybe youre sandboxed.
If all of the above doesnt give you the answer, then maybe study SEO