If you are using New Outlook for Windows with a non-Microsoft Account (e.g. GMail, Yahoo) then this is a Microsoft bug. There is nothing you can do from your Outlook Client to fix it.
See this Q&A thread on the same problem for more details:
https://learn.microsoft.com/en-us/answers/questions/5837171/contacts-in-outlook-365
If you are hitting this bug, your Contacts aren't gone, you just can't see them. But you should still be able to Export them to a CSV file for safe keeping. See the Answer by "Hornblower409 Mar 27, 2026" at
https://learn.microsoft.com/en-us/answers/questions/5818600/all-my-contacts-are-missing-with-the-new-outlook-h