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The answer to the question "why" is something you will need to investigate in detail. The obvious fact is that you were getting traffic from some specific search terms and for those terms you are no longer ranking. You need to identify which terms those are.
Your analytics, whatever kind you use, should tell you what those search terms were and what terms are bringing you traffic now. Investigate those keyword searches that dropped off.
The issue could be a technical thing, or it could be a penalty. You should have a Webmaster Tools accout set up with Google and check in there for information about whether Google is having technical trouble crawling your server.
If you can pretty much rule out technical trouble, then to uinderstand possible causes for penalties have a look through the topics in the Hot Topics area [webmasterworld.com], which is always pinned to the top of this forum's index page. Also keep an eye on the thread Google Updates and SERP Changes - August 2009 [webmasterworld.com].
All rankings stay the same or even higher..
So don't bother checking rankings yet, especially not directly at each data canter. The first job is finding exactly which keywords used to bring in the high traffic but no longer do.
we have 2 keywords that bring us almost 50% of traffic, until August the traffic they brought was rising and then stopped immidiately, their rankings remained the same
Seems clear to me that their rankings cannot be the same, unless the total volume of searches for those two terms has fallen off.
Maybe the rankings are now only for part of the day. Maybe there's some heavy geo-targetting making your rankings lower in most locations (but not the one you search from). Maybe the computer you check from is logged into a Google account. But if the actual volume on these searches is relatively stable, then the math should be clear - you've lost some ranking somewhere.
In which country are you? I'm in the UK and have seen some dramatic European traffic variations recently owing to many companies having the whole of August off on holidays...lucky so-and-sos to put it politely.
Please advise what to do if it is DNS hijacking? How can we fix it and what to check?
So,if it is seasonal
Well you'll definitely know next week, most of my clients return on 31st August even though that day is a holiday in the UK...the last until Xmas!