My iPhone won’t ask for the Wi-Fi password anymore but I need it to

If you are trying to access a protected Wi-Fi network whose password has changed:

Go into Settings, and tell your iPhone to forget the Wi-Fi network for which you need to enter a different password. Then join the network again, and the iPhone should prompt for a new password.

How to forget a Wi-Fi network on iPhone, iPad, or Mac - Apple Support

Is the Wi-Fi network protected?

If you have not configured security on the Wi-Fi access point / router, and the issue is that you want to password-protect the network, that's something you need to set up on the access point / router.