Over the weekend my Outlook categories have disappeared from the Categories list. How do I get them back?

We use categories to group emails coming into a Support mailbox. We set up lots of different categories for different people, teams etc. Each has a different colour assigned to it. When emails come in we will assign the appropriate category. Over the weekend, the categories appear to have been wiped from our categories list i.e. we now have only some predefined ones like Blue Category, Green Category etc. For emails previously categorised, I can still see what category I applied but the coloured 'sticker' is missing and I can no longer select those categories. In addition, if I set up a new category now, whilst I can apply a new category, it's not subsequently available for me to select in the category list, plus the coloured 'sticker' does not appear. Any thoughts on how we might resolve please? I'm using Outlook via Office 365.