Welcome to my world...
If you're facing this issue then it's most probably that your contacts are stored in Exchange and not in Gmail, Outlook.com, etc.
So the newest on this is that Apple have implemented a security measure to protect your contacts and are not allowing WhatsApp and other 3rd-party apps from accessing the iPhone's contacts, hence the empty WhatsApp contacts list.
The Apple folks are discovering it as we speak and will tell you "not my problem, check with WhatsApp", the WhatsApp folks are discovering it and telling you "not my problem, check with Apple" and the infinite loop syndrome is on.
Yes you can export your Exchange contacts and store them in Gmail or Outlook.com or even store them on the iPhone itself but no, I don't want to do that (or iCloud of course).
I want WhatsApp and other 3rd-party tools to recognise the need to support this new privacy protection mechanism.
Long story short, don't waste your time and wait patiently for all those guys to recognise the issue 😠
In the meantime, enjoy the alias given by WhatsApp to the contact speaking to you on WhatsApp (if any 😢)...
Tchoum999