If your Whatsapp is not working properly on your iPhone, then, the Whatsapp server, the old version, or the internet connection might be the cause. So, what you have to do is:
Update your Whatsapp application, if you are using the old version
Boost up the internet connection
Clear the cache data
And, one more thing is trying to reinstall the application
Read more: https://getassist.net/whatsapp-not-working-on-iphone/