If you lose your cell phone or it is stolen, all your information saved in WhatsApp
— including messages, photos, videos, etc. — will fall into the hands of strangers. Keep in mind that SIM card locking is not enough to close your WhatsApp session.
To fully protect your information, you should deactivate your WhatsApp account
How To Disable WhatsApp Account If Phone Is Stolen or Lost
In case your phone is stolen or lost, the first thing that you should do is call your mobile operator and request that your old SIM card be locked. If you have your phone's IMEI code, you can also request that the company lock the device itself.
After you've locked your SIM card, you should purchase a new phone with the same phone number and activate WhatsApp on the new phone. Doing so will block access to your WhatsApp account on your old phone, since you can only use WhatsApp on a single device at a time.
Another way to close your WhatsApp session on your old 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.
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 remain 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, they will automatically be deleted.
Image: © WhatsApp.