If this has happened to your encryption password / passphrase, read on...
Even though you cannot decrypt existing notes without the forgotten password, you can still encrypt new notes with a new password/pass-phrase. Entering any text in the data field (1); then tapping the "OK" button (2); repeat 4 times.
After the forth failure attempt, you will be give an option to set a new password/pass-phrase. Tap the "YES" button.
You will see a pop-up dialog box (1) for you to setup the password/pass phrase. Enter a very good hint to safeguard the passwords.
The passwords/hint for encryption of the old notes is not affected by this reset. The old password/passphrase for decryption will still work if you remember it at a later. Thus, to avoid confusion, try not to pick a new password/pass phrase that resembles the old one.