I used to have a lot of conversions, but recently CR has drastically decreased. What happened and how can I optimize my campaigns?

You should check first if your website or blog is still online and responding, so many users can visit it. Maybe your traffic sources have changed, or volumes have decreased, or your users come now from different countries than before? Also, if you made any changes on the site like changing banners and placements position, it can also affect your conversion rate.