Sorry to hear about this. There could be multiple reasons why this would have happened. Is the mouse working fine with other applications?
You may also try using a different mouse. If it shows the same behavior, we can try the steps shared on this page.
I´m using Windows 10 (2014 version, comp. 19041.572). And it happens while working with a fresh new document.
Plus, i´ve also found today that i can´t drag and drop files in after effects 2020 from the project bin, to the comp timeline -yesterday before update was working fine-. Maybe global issues with mouse drivers/shared libs?.