The CTR stayed the same as far as I can see. In other words, both impressions and clicks went down together. The problem is that the ads stopped showing. Why, I have no idea. Anyone have any?
type the query you would expect to see your ad appear on, copy the URL, paste it into the 2nd box of the diagnostic tool and it'll tell you.
I've experienced a few issues like that: my display url was too similar to another ad the was "ranked" higher than me, my daily budget / CPC needed to be tweaked, etc
HTH
(Besides, it's been running for several months without problems.)