I've recently installed LO ver 6.4. It's not opening at all.

Have seen this issue mentioned before, but I need a layman’s explanation as to how to get Libre Office ver 6.4 to open, please.

Without knowing your operating system, it is difficult, nay impossible, to give a useful response. Please edit your question to add this information, perhaps also with information on how you are trying to ‘open’ LibreOffice, and what exactly happens, including any error message(s).

If you are on Windows, have a look at General Installation Issues (Windows)