As a general practice, it is recommended that you keep backup copies of any completed notebooks. Just do “File → Download (as notebook)” from any notebook. Note that because of some browser issues, the file may end up with the extension “.json”. You’ll want to rename it to have the extension “.ipynb” if you ever want to upload and use it again. Coursera has been known to randomly erase student work for no particular reason, so it’s best to defend yourself in this way. “Fool me once, shame on you! Fool me twice, shame on me!”