Lost all previous notes after upgrade to El Capitan
Here's how to recover all our notes:


Open the terminal. We find it in Applications -> Utilities -> Terminal, or simply by searching in Spotlight.
Entered the terminal, let's move to the folder where the files where there are our notes by typing cd ~ / Library / Containers / com.apple.Notes / Data / Library / Notes / followed by pressing the enter key on the keyboard.
We initiate the SQLite command shell by typing sqlite3 and pressing the enter key. At this point we find ourselves on the message screen sqlite> . Great, we are prompt in the program.
We type .open NotesV6.storedata pigiamo and the enter key of the keyboard. It should not happen anything, but everything is normal.
We type select ZHTMLSTRING from ZNOTEBODY; therefore sending. At this point you should be displayed several lines of html code containing inside all our notes.
By scrolling through the results on the screen, select all lines of code that are present from the tag  to    and clicking the right mouse button on the selection, then choose Copy.
We type .Quit to exit the SQLite shell and close the terminal.
Paste the text you just copied into TextEdit and save the file in web page format (.html).
At this point, opening the .html file you just created, we will meet within it all our dear old notes that were lost.


-- Directions retrieved from here: http://www.maccanismi.it/2015/10/06/problema-risolto-note-scomparse-da-os-x-el-capitan/

I had some trouble after copy and pasting to text edit and then saving as html. What I had pasted was html content inside of html. So when I opened the newly saved html file in chrome, it just displayed the html code. Just copy and paste the information in the webpage to a new text edit document and save as html again. Now you can open it in and it will render correctly.