Hey there,
To unlock a note, you need to enter the password that it's locked with. If you forgot the password, Apple can't help you regain access to your locked notes. You can create a new password, but this won’t give you access to your old notes. A new password will let you password protect any notes that you create going forward with the new password.
If you have multiple passwords, you won't be able to tell which password you should use when you look at your Notes list. When you open a note locked by your old password but you enter your current password, you'll see an alert that you entered the wrong password with a hint for your old password. If you enter the correct old password, you get the option to update that note's password to your current one.
From this article ☞ How to lock or unlock notes on your iPhone, iPad, and iPod touch - Apple Support