Why is this happening? I need to know...

Long story short, i have over 30 web sites and several of them (around 9 or 10) have vanished from showing up on their google search terms and yet they are STILL indexed. We're not talking a drop in ranking, we're talking a complete removal. A couple have been in this state for almost 4 months, and the rest happened a month ago and are still gone. They are all indexed still, but not ranking at all for ANY search terms. Why does this happen? Will they ever come back? I've tried to continue backlinking to them but nothing is bringing them back. In fact, just 2 days ago yet another site went away, which was ranked #1 for its term and has been for almost 8 MONTHS (my oldest site), and never had a single problem until 2 days ago when it disappeared just like the others (but still indexed.) Here is some extra information about the situation:

1. All of these sites were on the first page of google for their terms. I have many sites not on the first page, but none of these have ever disappeared. I still have many sites on the first page of google still showing.

2. The same backlinking strategy was used for all of my sites. Nothing special, mainly article submission, some social bookmarking and blog comments.

3. I've tried adding more pages of content to all of these sites as well as more backlinks, and yet no results in almost a month since doing this.

4. All missing sites show up for a site:"http://url" search. They will also show if i type the url example.com without http or anything. Yet they won't rank for any search terms.

So what's the deal? Has anyone else had this happen? If so how long did it last? Is this some sort of "ban" on googles part or something? I just want to know why this is happening so i can fix the problem (if there is one), or whether its a normal thing that i just have to wait out. Naturally this is having a major (and apparently long term) affect on my earnings.