What should I do if my iPhone password isn't working?

What do you mean by problem? It is an all or nothing type of thing. It either works or it does not.

This Apple article has instructions for what to do if you have forgotten your Apple account password —> If you forgot your Apple ID password - Apple Support

or go directly to this link —> http://iforgot.apple.com/

If you don't receive your verification or reset email, read this Apple Support article: If you didn't receive your verification or reset email - Apple Support

"When you make certain updates to your Apple ID, we’ll send you an email to help you complete the update. For example, you might have reset your password, reset your security questions, or added a new email address to your account."

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple ID password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. More detail in —> Activation Lockout Issues - Apple Community

If you still need help this page provides country-specific Apple Support contact information for customers seeking help ➞ Contact Apple for support and service - Apple Support

This page has telephone numbers for contacting Apple for support and service ➞ Contact Apple for support and service - Apple Support

If you are signed into this forum using the same Apple Account then your browser may be configured to automatically enter your password. You should be able to view stored passwords in your browser's preferences/settings.