QuickBooks Error 6073 on a single-user mode can occur due to various reasons, including damaged or corrupted company files, incorrect file location, multiple users trying to access the same file, outdated version of QuickBooks, and firewall or security software blocking QuickBooks access to the file. To fix this error, you can use QuickBooks File Doctor, check the file location, update QuickBooks, disable the firewall or security software temporarily, or restore a backup of the company file. Regularly maintaining backups of your company files and keeping your QuickBooks up to date can also help prevent this error from occurring in the future. It is important to take necessary precautions to prevent such errors from occurring, such as regularly updating QuickBooks, maintaining a backup of company files, and using reliable antivirus software to protect the system from malware or virus attacks. By following these steps, you can ensure that your financial data is secure and accessible at all times.