sounds like it may have been deindexed - have you been using any black hat methods for your SEO? I may be able to help you further if you pm me your email or further details.
Otherwise if you'd prefer to look into it yourself I would start by looking through Googles guidelines and see if you have done anything that they seriously disagree on - I find this way you can usually figure out what might have caused it if it was self inflicted and fix it a lot quicker.
Once you have found out why your site has been indexed - and fixed it, you should go into your google webmaster tools and request your site to be reconsidered & explain why they should reconsider you.
If your site is spammy, or built solely for the purpose of advertisements - this could also be a reason your site was deindexed.
Don't bother asking them to reconsider your site unless you have made some kind of ammendment to what may have caused it in the first place - If it is the case that your site was deindexed.
EDIT - your right Chickenboom, you do write site:whateverdotcom but if your site is indexed on google it should show when you enter your Url address, site: is more accurate though!
Hope this helps you recover,
Kindest regards
Tom