If QuickBooks won't open, there could be multiple reasons such as system issues, corrupted installation files, outdated software, or damaged company files. First, ensure that your system meets the minimum system requirements and restart your computer. Try running the QuickBooks Install Diagnostic Tool, which can identify and fix installation issues. Use the QuickBooks Refresher Tool to close and reopen background processes associated with QuickBooks. If the issue persists, try opening QuickBooks in compatibility mode or reinstalling it.