With the traditional Notes password lock, Apple does not save the password anywhere, they use the password as the data encryption and data decryption key. No password, no decryption.
The cryptography also uses robust and secure algorithms with no known back doors.
If you do not know the password, you are indistinguishable from an attacker seeking to access the protected data.
The one thing that does work here are guesses. Unlike most other contexts with passwords, infinite guesses here are permitted.
Look up other passwords you created around the same time as potential reminders.
Look at the password hint, if that was specified.
More recent versions of Notes on iOS can optionally use the device passcode as the Notes passcode, but that has to be selected when the Note is created.
Some background info: Secure features in the Notes app - Apple Support
