Kristina has good questions. You need to isolate the problem metrics as best as possible. In GA you can parse out influence of click bids vs. CTR having the effect on RPM/eCPM.
If it's a CTR issue that is responsible for 100% of your ad revenue drop and eCPM drop, ie. the click bids went unchanged, then you need to look at Mobile vs. Table vs. Desktop changes in CTR over the period in question. You might want to take a look at CTR changes based on source/medium as well and drill into anything that seems significant. Some of the adsense metrics in GA are a bit suspect. I like checking earnings per click against adsense revenue / adsense ads clicked.
We noticed a big drop in mobile ad CTR cutting our ad revenue in half, disproportionately affecting iPhones and it was due to the change to HTTPS having an effect on who our advertisers were indirectly affecting ad relevance/quality. Desktop CTR actually went up but the overall decline hid the problem from the client.