The goal of marketing analytics isn’t to simply understand what has happened, but to learn “why” it has happened. This understanding of “why” is what we call a marketing” insight”. That should be what all your monitoring and measuring are focused on, learning why a metric has changed or is different from another segment.
Looking at aggregate data “averages” tells you almost nothing. You need to segment your data to see which segments have changed. For example, traffic from some keywords might have remained unchanged while other keywords have tanked. Traffic might have dropped from many keywords all at once, or at different times for different keywords. That tells you something useful. You can start looking at the specific terms that have changed the most and focus on one at a time to first understand what has happened and then to dive deeper to try to gain an understanding of “why” it changed.
Search trends may have changed, some regions may have changed while others have not. Perhaps, the changes coincide with the new content you posted? Perhaps your average page load speeds slowed just before search traffic dropped off? The bottom line is that there are a bunch of stories that your data can tell you if you just dive deeper to see what is happening at a micro-level. Those micro-stories will give you a better understanding of the macro story that currently has you puzzled.