https://downloads.quickbooks.com/app/qbdt/products?productName=QuickBooks%20Mac%20Desktop&edition=&v... is the download page for 2020. Download it, drag it in your applications folder. Double click on it in your applications folder (this avoids your doc pointing to somewhere else). Hold down the option key while double clicking on it - this prevents it from loading your last used company file.
If QuickBooks does not launch after doing that there most likely is something on your machine that is preventing it. What macOS version are you running? If it is not supported that's the issue. If the macOS is supported then the issue is likely some driver, system extension, bad font, or foreign typing system that is interfering with QuickBooks launch. You can test that by "Safe booting" your Mac:
https://support.apple.com/en-us/HT201262
If QuickBooks loads while in "safe boot" mode then the issue is one of the above.
Let me know what you find.
