Need a lot more info. You need to find data that answers the following questions: 1. Any major changes/events within the past 6 months? (or, is your product still as good as it was 6 months ago?) 2. Any new competitors? Any existing competitor had a major update/upgrade on their product? 3. How is the industry your product serves doing? 4. Any (strategy or metrics) changes on marketing? 5. How are your customers doing?

Also, it helps a lot to schedule some calls with customers who left.