Hello Mike-BC,
In addition to what user Axel Foley said, trying to guess the password may be easier with some additional info.
Individual locked notes do not have unique passwords. The password you created would have been for the notes account that the note is in. (i.e. iCloud or On My iPhone). If you make a security change (reset password or enable Face ID) you need the old password to apply that change the next time you unlock each note in that account.
Some people reuse their device or Apple ID password so make sure to try old device or ID passwords from when you first locked a note in that account. If you previously reset the password but infrequently unlocked notes you might have multiple old passwords in play.
If you can access some locked notes with Face ID but do not know the current password make sure to follow these steps found in the “Learn more” link below “Use Face ID” in Settings > Notes > Password, before resetting the account password.
“If you forgot your Notes password but can still access locked notes with Face ID or Touch ID
Do the following to set a new password:
1. Open each locked note and copy the
contents to a new, unlocked note. This
includes locked notes in the "On my
iPhone" account if they use the same
Notes password as your iCloud
account.
Important: Make sure you do this for allyour locked notes. After you do the next step, you can't access them with
Face ID or Touch ID.
2. Go to Settings > Notes > Password,
choose an account (if necessary), tap
Reset Password, then follow the on-
screen instructions. Be sure to add a
hint so that you don't forget this
password.
3. Lock each of the notes that you created in Step 1. Since you can no longer access the original set of locked notes with Face ID or Touch ID, you can delete them to avoid confusing them with the new set of locked notes.”
I hope this helps.