This seems to be an ongoing issue with the Windows 10 upgrade.
The unfortunate thing is that no technical person has yet had this happen so they can diagnose the issue.
You will find some threads in the calibre Devices subforum discussing this; see
https://www.mobileread.com/forums/sho...d.php?t=264201 https://www.mobileread.com/forums/sho...d.php?t=263599 https://www.mobileread.com/forums/sho...d.php?t=263743