QuickBooks Connection Has Been Lost error occurs when QuickBooks loses the connection to the company file. This issue can arise due to network problems, server issues, or file corruption. To resolve this error, ensure that your network connection is stable, restart the QuickBooks Database Server Manager, and verify that the company file is stored in the correct location. Running the QuickBooks File Doctor tool can also help identify and fix any file corruption or network issues. For a detailed guide on resolving QuickBooks "Connection Has Been Lost" error, visit Guide.