A quick Google suggests you may be able to open the memo and reset it to "public" (which turns the password request off) by using "a third-party text-editing application, such as RsrcEdit, Hotpaw BASIC and PEdit". Worth a try, anyhow...
You must have set a passcode sometime, because memo will not ask for a private passcode unless you set one. May be you have a third party program like teal lock set. I just checked a memo as private, and it would not open unless I entered my teal lock unlock code.
I just truned off my teal lock, and made sure the palm has no password assigned, and a private record still asked for a passcode, and the teal lock code worked??