De-indexed means that your site was removed from the google index. If your site does not show up for its main keyword, it DOES NOT mean your site was de-indexed. Even if you search for your domain name and your site does not come up, this also does not mean you have been de-indexed.
To see if your site is in the google index you search for this "site:yourdomain.com" making sure not to put a space after the ":". If that does not bring back any results, then you actually have been "de-indexed", assuming you were indexed to start with.
Sent to hell is one of the popular terms used is SEO community to describe when a site still exists in the index, but dissapears down the rankings to a page where no one will ever see you. This is usually due to google disregarding a large portion of your links or applying a ranking penalty of some sort.
I have done several experiements trying purposefully to get a site de-indexed, just to see how far you have to push it before it happens. Contrary to popular belief it is pretty hard to get a site de-indexed. I certainly think the chances of it happening by accident without someone purposefully doing something dodgy to effect their rankings are extremely remote.
The 3 surefire ways to get your site de-indexed I have found are:
1. Multiple subdomains on same domain with automatically generated content. BlueFartters have been doing this trick for years. They get a domain then keep spawning autoblog subdomains off it with generated content and then interlink the sub domains. I also did an experiement with addidng reciprocal links directories to sub domains and had the reciprocal link point to my main domain. This also got me de-indexed.
2. Cloaked content. People use rubbish autogenerated content to display to search engine spiders, but display CPA offers to humans. This will get you de-indexed for sure.
3. Interlinked network over multiple domains. This one is harder to get de-indexed for. Generally if you interlink loads of unrelated domains in an obvious pattern, leaving obvious footprints then google can manually remove the network from the index if they deem it to be spam.
What usually happens with interlinked networks when people start screaming "OMG ALL SITEZ R DEINDEXED", is that they were ranking due to the fact of all the interlinking between sites they had done. The google algo then detected the obvious pattern of interlinking and chose to disregard the links. This of course reduced their ranking power and sent them to hell. The sites still exist in the index, they just dont rank as their incoming links have been disregarded.
I have been doing SEO for a long time and personally test everything before I comment on it. I have done all of these techniques just as an experiement to see what happens. I have never had a site de-indexed by accident in 8 years of optimizing sites.
If your site does go to hell it doesn't mean you should panic. They usually come back given time, some new content and some quality incoming links.
If you are going to base all your income in settining up SEO networks then you should create multiple networks, multiple accounts, mask your domain whois and hide obvious fingerprints. This way you wont run the risk of loosing your whole network in one swoop.
Getting de-indexed is rare, even for interlinked domains. Google usually just dis-regards the links so the sites don't rank well. Google generally looks to de-index automatically generated content or cloaked pages. As long as you don't put out rubbish pages or use BlueFart scripts you should be ok.
I could put 100 pages on 100 domains and interlink them. As long as all of those pages were good information then Google has no reason to de-index them, although if I interlinked them all it would leave an obvious fingerprint and it would just be a matter of time before those links were disregarded.
Does your data show different? I only have my own data to base this on and the people I work withs data so if you have some evidence to the contrary then it would be good to hear from you as I am in process of setting up several networks at the moment.