If you lose your cell phone or it is stolen, all your information contained in WhatsApp (messages, photos, videos, etc.) will fall into the hands of strangers who may misuse it. Keep in mind that SIM card locking is not enough to close your WhatsApp session. The application can connect to the Internet over Wi-Fi and the stranger can see the messages you receive and even send messages to your contacts.
How To Disable WhatsApp Account if Phone is Stolen or Lost
In case your phone is stolen or lost, the first thing to do is to call your mobile operator and request the SIM card to be locked. If you have the IMEI code, also ask for the cell phone to be locked. After you've locked your SIM card, purchase a new phone with the same phone number and activate WhatsApp. This will block access to your WhatsApp account on your stolen phone, since you can only use WhatsApp on a single device at a time.
Another way to close your WhatsApp session on your stolen device is to temporarily deactivate your account. To do this, send an email with your phone number, including your country code, to the WhatsApp support team.
Please note that even if your account is disabled, your name will still appear in your contacts' contact list. Your contacts can continue to send you messages that will be pending for a maximum of 30 days. If you reactivate your account on another device before this period expires, you will receive the pending messages. Otherwise, these will be deleted automatically. You will, however, not be able to retrieve your chat history and previous messages since they are stored on the device.