There are several reasons why QuickBooks won't open. Some common causes include corrupted company files, outdated software versions, damaged installation files, and conflicting software or hardware. To resolve the issue, try running the QuickBooks Install Diagnostic Tool, updating to the latest version of QuickBooks, restarting your computer, or repairing the installation files. If none of these solutions work, you may need to seek professional assistance or contact QuickBooks support for further guidance.