It's not easy to answer that question directly since it is broad. What could happen is they might be able to steal some information from your WhatsApp account (which is independent of your device). Now, if you happen to have the same password for your WhatsApp account and your Microsoft account, then they could attempt to access your Microsoft account especially if you don't have two-factor authentication active.
However, that still doesn't mean they can directly access your phone. Even if they get all your contacts, all your installed apps, all your personal info, they still can't get into your phone. They could almost clone your phone, but they couldn't have hacked your actual phone. This is because of the security details built into Windows Phones.
So the short answer is: No. Despite the alarming possibilities I mentioned, those are all improbable. Unless you're a target of some international organization, you most likely won't have to worry about anything.