The password is stored in your keychain. To recover the password follow these steps:
1. Open the 'Keychain Access' app - it's located in /Applications/Utilities (or search using Spotlight)
2. In Keychain Access, search for 'Sticky Notes'. You should see a single item named 'Sticky Notes' with the type 'Application Password'.
3. Double-click on the Sticky Notes keychain item. You should see a detail sheet.
4. In the detail sheet, check the box labelled 'show password', you will be prompted for your keychain password (the same one you log into your Mac with). Your Sticky Notes password will be displayed in Keychain Access.