When QuickBooks migration fails unexpectedly, it's essential to take prompt action to address the issue. Firstly, document any error messages or symptoms encountered during the migration process. Next, attempt to restart the migration process to see if the issue resolves itself. If the problem persists, review the migration settings and ensure compatibility with the target platform. Consider contacting QuickBooks support for guidance and troubleshooting assistance. Additionally, check for any updates or patches available for QuickBooks software that may address migration-related issues.