QuickBooks Error 6123, 0 is a common issue that occurs when attempting to open or restore a company file in QuickBooks. This error message typically indicates a loss of connection to the QuickBooks company file, potentially caused by network issues, corrupted company files, multiple instances of QuickBooks Database Manager, or conflicts with firewall or security software. Identifying the specific cause is crucial for effective troubleshooting. Solutions include using QuickBooks Tool Hub, renaming .ND and .TLG files, disabling security software temporarily, or restoring a backup file. Addressing this error promptly ensures seamless access to your financial data and uninterrupted business operations.