Started with Windows 8.1 and it worked... for a while.
Upgraded to Digital Editions 4.5 - problem started.
Uninstalled Digital Editions and reinstalled -same problem.
Rolled back to an earlier version - same problem.
Erased authorization and reauthorized - same problem
Upgraded to Windows 10 - same problem.
I can sometime access the downloaded books in the folder where my computer stores them - but the "library" never populates.
Closed Digital Edition.
Rename the My Digital Edition folder under your Documents to My Digtal Edition.old.
Start Digital Edition new.
OMG - Brilliant and it worked beautifully!
You are welcome.