About a year ago, I set up a simple product page on my website that describes my product and contains a download link, and I added a AdSense block to the bottom. During that year, all key metrics (number of visitors, impressions, active views, clicks/CTR) for this page stayed the same, and so has the AdSense revenue.
But there has recently been a sudden drop in clicks/CTR, by about 50-70%, and a subsequent drop in revenue. It has been happening for over a month now with no apparent reason: the page content has remained the same, the number of visitors and their device types has been constant, I haven't touched any AdSense settings, my product is not seasonal in any way...
How do I find out the reason for this sudden drop in revenue?