My Whatsapp is hanging and crashing on my iPhone 7 iOS version 14.4.2

Try forced restart

https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

Of that fails, backup message history to iCloud using WhatsApp’s own backup procedure, uninstall the app, reinstall the app then restore history from iCloud in Whatsapp

WhatsApp is not an Apple product. Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp