I managed to get my missing notebook restored with help from Evernote Tech Support, eventually. Here are the instructions that I followed, my own details replaced with <>. Hope this helps.
I reviewed your account and it appears that the references missing notes may not have properly synced to the servers from the Evernote for Windows application v 6.26 prior to update (between 2020-09-22 and 2020-10-21). With that being said, there should still be the local database from your previous version stored on your PC, and we should still be able to recover the missing content from t