Click on this link to read 'How to lock notes on your iPhone, iPad, and iPod touch' --> How to lock or unlock notes on your iPhone or iPad - Apple Support
Reset your notes password if you forgot it
To unlock a note, you need to enter the password that it's locked with. If you forgot your notes password, Apple can't help you regain access to your locked notes. You can create a new notes password or use your device passcode instead, but this won’t give you access to your old notes. If you set a new notes password or switch to your device passcode, you can protect any notes that you create going forward with the new password.
If you have multiple notes 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 an old notes 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.