My site, http://www.pesinsight.com, used to rank number one for ALL the important keywords. Suddenly one day, around 6 weeks ago, that all changed and my site was dropped around 40 places for EVERYTHING - and that includes the most obvious keyword 'pesinsight' - which I currently am down around 50th for. Thankfully another domain owned by myself ranks number one for 'pesinsight' - so people still do come accross my site when searching for it specifically. But what caused such a drop? Especially on the pesinsight keyword.
Maybe you have done this (1) Linked to a bad neighborhood, that is, a site that is thought by Google to be bad. (2) Did something else Google didn't like (hidden text, too much keywords on one page) IT
try copy / paste - forums.digitalpoint.com/showthread.php?t=5041&highlight=robots.txt while it may be noted in the thread, that you don't technically need one, yours is showing a server error. Best to get it fixed in my opinion. Had a new site recently that DIDN'T have one and Yahoo seemed to be hitting just for that file and then went no further. After uploading a robots.txt, the bot presumed to be welcome and started indexing. Hope you get the issue resolved swiftly.
I think you need to concentrate on keyword prominency, proximity and density. Which is the main reason for not coming on top pages of SE.
That's not what has happened though. My keywords are fine. Looking at "pesinsight" keyword particular. It is the SAME site effectively which is top for that keyword (pes-5.com) as what is on pesinsight.com. Yet pesinsight.com is way down at 50. It happened randomly over night over a month ago.
your site pes-5.com redirects to pesinsight.com/Pro-Evolution-Soccer-5.php. therein might lie your problem. if redirects are not done correctly, google can penalize sites for this. it doesn't look like it's redirecting right, but i don't know a whole lot about redirects. that's my uneducated guess. someone that knows more about redirects could help you better.