I have never had this problem before with Adobe Digital Editions, but since yesterday the books that I download do not appear in ADE. I have tried several times to close the program and try again, I restarted my computer, but the same problem remains. I tried downloading different books, the same way I always do, and it still doesn't work. I have Windows 10.
Another member posted the same questions a few years back, and someone gave the following answer :
Closed Digital Edition.
Rename My Digital Edition in My Digital Edition.old under My Documents. Restart Digital Edition.
Open the eBook again.
This seemed to have resolved the problem for them but I have tried doing the same thing and it doesn't work.