If your contacts' numbers are shown instead of their names, you might need to reset the WhatsApp sync with your contacts. To do so:
If the steps above don't work, it's possible that your phone's settings might be preventing WhatsApp from functioning properly. If so, please try the following:
Open your phone's Settings, then tap Apps & notifications > WhatsApp > Permissions. Make sure all permissions are turned on.
If the issue still isn't fixed, please contact your phone's manufacturer for more information regarding security permissions on your phone.