If your listing in Google is suddenly gone, it's possible that your site was flagged as having duplicate content. This can trigger Google to suddenly penalize your ranking and possibly drop you altogether.

An example of duplicate content is having identical text on multiple pages of your site or across various websites. If this is the case, it is most likely 100% innocent on your part; you just have to convince Google of that.

To check your site for duplicate content, follow the advice at Hobo-Web.

You can also enter a URL of one of your website pages to CopyScape.

The URL may be one like this:
http://(YourDomain.com)/about

If you find for example, that another website has copied some of your bio (such as a gallery or online magazine), simply re-word your bio so they don't match. That may help.

For more info, see:
Support.Google
Duplicate Content Problems