Seems like the same thing might have happened to me last week. Something has changed and I see everything again. I did nothing to make the change . . . it just happened.
I don't know whether I now have the web version or the app version. I probably always had the app version, because on my landing page I always have clicked Apps to get to the Outlook mail icon. GoDaddy told me the problem was Microsoft when I called an agent a while ago. On the left side of the Outlook People screen, I now see two sets of Contact lists, but under slightly different folder names. Top to bottom, I see these folders: Favorites, Your contacts, Your contact lists, Deleted and Folders and Directory and a blue link, Go to Groups. The Folders icon has a dropdown arrow that reveals these subfolders: Contacts, New Applicants and a link, Create new folder. I've never seen two sets of folder trees. Clicking on Directory reveals Contacts and other folder, all of which are empty. except for Global/Default Address List and All Users.
Both sets of contact lists have all of the names that should be there, but they appear twice in each contact folder. The name-doubling is likely because I followed a suggestion I saw online and created the contact list by copying and pasting the names from my Sent folder, then uploading/importing them to Outlook, but I don't recall the steps. At the time, I saw no change and the contacts were still hidden. Meanwhile, I've been sending to my contact lists as usual (their names appear when I start typing), and everything has gone through (no duplicate emails going out, thankfully). As stated, the list became visible again last week. Microsoft must have quietly populated some system fix or new way of doing things in the background. Anyway, all seems well now.