A password protected note refuses to open - says password is wrong (it's not).

[AC Question]

I'm using S Note (Samsung Galaxy S4, not updated to Lollipop) to keep track of some important info. I had not backed up yet. I had a very simple password that I've used plenty of times to open this note. This morning it told me the password was wrong. I tried checking to see if shift or caps lock was on, I tried to see if I could have mistyped it somehow, I tried everything. Why would it suddenly say the password was invalid when I've used the same password for well over a month?