You probably have a Google penalty when your site (or an important page) suddenly disappeared from the index. You will recognize this by lower traffic and lower sales.
But if your site had a steady ranking (for a longtime) for a specific keyword phrase and suddenly the usual ranking drops ten or twenty pages, then this also sounds like a penalty.
But to be certain about this, first do some other ranking checks and visit the most important Webmaster forums and blogs, read some daily SEO feeds to see if this signal has happened to others too. Sometimes during a Google Dance (the moment Google is reindexing his servers), a large rise and fall in rankings can follow for a few days.
A new algorithm implementation can also have this effect.
But if there is no explicable reason for a large drop down (or a removal from the index) of your ranking, then you should think about a possible penalty.
The first thing you should NOT do is contacting Google immediately.
If you have a Google Webmaster account then they probably will contact you with a message if your site has been manual excluded from the index.