Can't see or edit contact lists suddenly

Hi, I am not super techy but have used without problem the contact lists in outlook (logging in on a browser) for years. Suddenly, I can't see my lists anywhere, but if I write a new email, I can still select a contact list in the address bar. However, if I click the 'people' icon now, I only see my address book as a list of alphabetical contacts. Can anyone tell me what to do? I am using a MacBook. Thanks