I have website since 2007 and 90% of my visitors is Google visitors. 2 months back i lose 85% of my traffic without any reason. I don't have any message in Goog

I have website since 2007 and 90% of my visitors is Google visitors. 2 months back i lose 85% of my traffic without any reason. I don't have any message in Google webmaster. Google is applying filters to reduice my traffic on search results. Sometime it go to normal so i receive all my visitors and 2 days after penality comeback. I don't undertand how google works and i am not using any spammy links or other. So google is kelling the web ....