This discussion at Apple support offers some hints: https://discussions.apple.com/thread/3070330?start=0&tstart=0

You can still use the app, by showing the categories in the main window (use the "show" link next to the word "CATEGORIES"). After you click on one category you'll be able to browse entries also under the "All entries" item.

The developer's website seems to have vanished, so don't expect updates and plan for data migration (you can export each category to CSV).

H there what happens if you cannot even opeN xhead and get to the categories you mention. How can i find the data file .
Thanks for that

Workaround for "opening database" hang:

For me the main menu still works, choose File > Backup Database > Save it somewhere then the app works fine.

Time to export and find a better solution though, I'm thinking simple txt files in an encrypted disk image.