I'm not sure how it happened, but the Kindle desktop software fouled up my computer so badly I had to run microsoft's restore option to reset the computer, removing all of the software I had installed over the years. Luckily my data & files were all backed up safely, but finding the installation files, license keys and reinstalling the software has been a time consuming pain.
I will throw kudos and many thanks to Foxit PDF Editor customer support, who helped me get the application reactivated on my computer - it's a really old version of their software, probably considered obsolete by today's subscription only standard, and I am so thankful for their help.
I will not install the Amazon Kindle software to my compter again...