Getting indexed by the search engine means the search engine has "seen" your web page and put it into their database. De-indexed means they deleted your page out of their database (of "good" pages).
To check if you're (de)indexed on Google you can type in:
site:www.yoursite.com

If no results come up (and the site was indexed before) - it's probably a case where they de-indexed you (for violation of their Terms).

If I see my site is still there when I type site:mysite.com but nowhere within top 100, that could be mean my site had google dance? It was on page 1.

This thread will surely help you -

http://www.warriorforum.com/adsense-...e-indexed.html

[DELETED]