Google tends to remove sites from their main index sometimes. That's probably what happened to you... You just have to wait and it'll get back to normal.
How old is your site? I have experienced this with new sites, especially if I have been a little over zealous in building links. Also, how long was it ranking well for? It is not unusual for your rankings to go up for a short spurt and then go down again. I have found it can take at least a year sometime more of steady SEO for SERPS to become reliable.
my site is almost 3 years old and have extremely unique content with no stuffing keywords. I have applied only white hat SEO to optimize it! It shows me sometime in first page, sometime in second and sometime no where in first 100 position!
My site bounced out to 60 for my choosing keywords but is now back in the top 10, google does this at times and you just have to wait your time, while doing so i suggest you build backlinks, doing this can't hurt.
On checking my website ranking, I found that one of my keyword was ranking 6th position in Google.co.in, but in afternoon, when I rechecked it fall down to second page....I don't understand what is happening!
temporary thing . i also got the same problem . my site pagerank got 1 from 3 but still have no1 rank in google.
Well the same thing happened over here last week for one of my websites! My SERPS dropped from 1st/2nd page to the 100th page .. It was kinda funny for me since with this loss of SERPS I got a higher PR on the recent PR update. It only took 5 days before my SERPS returned back to normal. I guess this was a Google dance to test out some algorithms or something... Everything will be fine, don't worry
I see that you have 70 internal links on your homepage that seems far too many in my option try reducing this