WhatsApp is not working on my iPhone 5s

Delete WhatsApp Messenger, force-restart your iPhone, re-download the app, and check again.

Then check with the app vendor:

… Contact a third-party vendor - Apple Support

You will need to contact WhatsApp for help with their software, this is not an Apple product.

Here is a link to their help page.

https://www.whatsapp.com/contact/?subject=messenger