Do not panic! There are many possible reasons for this...and not all are horrible (honestly). In order to create a recovery plan that works, your data investigation should take place in a thoughtful and systematic manner.
The "investigation Process" boils down to the simplest steps:
Validate that your problem is not a traffic one.
Learn from your data: Where does the drop originate?
What is its magnitude and duration?
How is your site making an impression?
Identify the root causes, and then find the solutions.…