WhatsApp not opening on iPhone 6 for past few days

First, try a forced restart. If that doesn't work, uninstall and reinstall the app. If that doesn't work, contact WhatsApp for help with their software.

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

WhatsApp help -

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

The solution is to contact WhatsApp. This is not an Apple product.