Case 1:
If you find yourself getting logged out and have returned to the Title Screen - there is a "Restore from NeoPass" button at the top left on the Title Screen. Click it to log in and continue the game where you left off.
Case 2:
If you accidentally started a new game (Level 1) from the Title Screen, you can go to the game Settings after the tutorial and (a) either delete the guest account (click the Bin icon) to go back to the Title Screen, or (b) log in to your NeoPass (click "Link to NeoPass") and overwrite the guest progress with the existing progress in your NeoPass.