**SOLVED!!**π
... *Hopefully for most of you, too!*π€ *Note: Earlier suggested solutions on this thread, prior to May 2024, involved options discontinued by Microsoft, or just stopped working!*π
Short answer: Check your "Contacts Privacy Settings"!
Microsoft may have set the desktop People app's access to "Off" during a forced contacts/mail migration campaign (see screenshot #1 below)--*feels more like war!*π°

Screenshot #1
It makes it look as if the People functionality no longer works on your Desktop Mail App. Btw, Microsoft CS refused to tell me if a download of the People app was possible.π
Set the option to "On" (see screenshot #2 below), et voilΓ , your Contacts will hopefully reappear and your desktop People App will work as before (see screenshot #3 below). Worse, if you denied access to Web Outlook during that migration, your contacts won't show up there either!

Screenshot #2

Screenshot #3
Note: you may still have to contact Microsoft Customer Support (CS) and ask them to sync your Contacts database.π
I'm guessing they did something like that, but the CS supervisor, A** T****, stubbornly and repeatedly refused to tell me what he did. He certainly unlawfully enabled Web Outlook to have access to my (server-based) contacts, even though I'd specifically denied it--still want control centre to be my desktop Mail app--and hadn't granted any such authorisation to CS!π Worse, CS supervisor, A** T**** rudely terminated the connection before I could submit a much-deserved negative review of his 2-hour stubborn refusal to even read my questions, let alone answer them, nor accept that his apparent limited experience was never going to be good enough to understand, let alone solve this thorny problem of apparently destroyed Contacts!
Note: When you enter/edit contact details, the server backup/synced database should be updated accordingly--pointless without access via an app. I'm still not certain the server database contains all of the fields I used on my desktop People App over the years or that it is instantly synchronised.π€¨ CS supervisor A** T**** utterly refused to do anything that might prolong my use of desktop Mail!π Basic incompetence? Feigned ignorance? Official policy?π€·ββοΈ
πTip: As this nightmare of a forced migration--to a poor version of the old cumbersome Hotmailπ--continues, you may want to regularly "Export" backup copies of your contacts database to local CVS files you can either take to a non-MS App or safeguard from another MS forced migration snafu.π€¨
Note to Microsoft: You've been aware of this problem for at least 6 months and have refused to respond to or engage with suffering users, yet disdainfully ploughed on with this forced migration to an inadequate app that is nowhere near ready! That is the height of arrogance and is utterly unacceptable!!π π π π π