Typical problems with notebooks, together with their solutions are listed here.
Enable extension for your jupyter:
jupyter nbextension enable --py neptune-notebooks
Click on the n button in your jupyter menu.
Click on Checkpoint.
Bottom drop-down is your current project.
Log in to neptune.
Click on your avatar (top-right part of the screen) and select Get API Token
Most likely, you restarted kernel. Here is a solution:
Go to configuration (n button).