There are several possible causes of the 'QuickBooks won't open' error, including problems caused by missing or damaged files inside the QuickBooks application, or by out-of-date drivers. You'll need to update your QuickBooks driver files by visiting the QuickBooks website. Another option is to reinstall QuickBooks to restore the lost files. Another improbable scenario is that antivirus software or a firewall is preventing QuickBooks launch. Typically, the firewall will do this if it believes Quickbooks is dangerous software. However, before blocking, it may request permission, which you must provide.