At first, you'll lose access to your chat list. The only way the app will allow you to message or call someone is if they contact you first and you tap on the notification from WhatsApp. After several more weeks, the app will stop sending messages and calls to your phone altogether. The exact moment the process starts for individual holdouts will be different for everyone.
Once you get to the second stage, your options are limited if you don't accept the policy at that point. You can export your chat history or download a report of your account. You can also delete it, though WhatsApp warns that's something it can't reverse. Ultimately, it may not matter much. As part of a separate policy, WhatsApp automatically deletes accounts after 120 days of inactivity.