Huge sudden ranking drop in google, how to find out why?

Until around September 2015 a year old website i made for a client was very highly ranked in google.ch (switzerland).

If you google-searched for a name of someone thats listed on the page, this page would usually turn up first, or in the top 3 results.

Suddenly google rankings dropped drastically and didnt recover since then. Most results now turn up on page 2-3, instead of result 1-3 of page one. IF they even show up, now sometimes the obituary sub-page of a person doesn't get indexed at all.

Roughly around the same time the rankings dropped, we added links for each city, instead of beeing available just through a drop down menu. But i cant say if it was caused by this, or was just coincidence. After about a month or two we reversed the changes and waited again, but it didn't change the ranking.

In february i changed the sites link-structure to make it more readable, and optimized everything that i thought "might" be an issue, but it again didnt change anything for the ranking.

I tried contacting google, but they never replied.

I wonder if there's a big elephant in the room that i can't see, or how you experts would go about in this situation?

Thanks for reading