Conversion rate dropped by 50% - use Google Analytics to find out why

Hello, One of my clients sites suffered 50% drop in conversion rate within one moth and We want to find out why did it happen. Your job will be to identify root cause of conversion rate drop. You will be given access to Google Analytics were E-Commerce tracking is enabled and history data is available. We'll also work with You and try to provide any help You might need to finish the job. To this date We excluded following reasons for conversion rate drop (however You're free to investigate them again.) - page speed, - change in layout / design, - issues with checkout. Job will be completed when You'll find cause of lower conversion rate and We _confirm_ it (by reverting back any changes made to the page and observing conversion rate over one-two week period). Please keep in mind that when You bid - You will not get paid until We confirm that conversion rate is back to normal. Also please keep in mind that We're not looking for new improvements to our site. We need to find out WHY conversion rate suddenly dropped.