My phone was stolen. How can I access my iCloud if I forgot my password

Depending in what you do remember of your credentials, and what recovery paths you still have access to:

If you forgot your Apple ID password - Apple Support

If you forgot your Apple ID - Apple Support

How to use account recovery when you can’t reset your Apple ID password - Apple Support

Contact Apple for help with Apple ID account security - Apple Support

As for a stolen iPhone:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

You;re probably going to be discussing this directly with Apple.

To locate your phone, go to https://icloud.com/find

You MUST include the “/find” on the url.

Sign-in with YOUR AppleID and Password. (No 2FA verification code is required to find your phone)

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To access “the rest” of your iCloud (assuming that you don’t have access any other “Trusted Devices”) …

… you’ll need to receive your 2FA verification code at one of your pre-designated “Trusted Numbers”.

See:

Get a verification code and sign in with two-factor authentication - Apple Support